I kinda found a temporary way to fix my issue, but I found a post really similar to my issue here [Stable Update] 2020-08-28 - Kernels, Systemd, PAM, PAMBASE, KDE-git, Deepin, Pamac, Nvidia 450.66, LibreOffice 7.0 - #73 by Hanzel
His issue mysteriously was there for a moment then it worked at some point after the non sense he tried like me.
I found what changed in the past week on my Manjaro, for a thread I replied to, I changed my DNS server in Network Manager, I had set cloud flare DNS servers, everything was working as expected for days.
The issue was not about the DNS server at all, BUT by manually setting the DNS server as 192.168.1.1 (the router IP address which was automatically set in theory with full AUTO in N.M.) then it connects properly, but here is the issue in log, like the guy above:
[omano@omano-nvme ~]$ journalctl -k -p err -b -1
-- Logs begin at Sun 2020-09-13 02:58:12 CEST, end at Fri 2020-09-25 21:35:51 CEST. --
sept. 25 16:32:56 omano-nvme kernel: sp5100-tco sp5100-tco: Watchdog hardware is disabled
sept. 25 16:33:25 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:35:12 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:35:22 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:36:28 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:36:53 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:37:28 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:37:53 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:38:28 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:38:53 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:39:22 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:39:32 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:40:57 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:41:32 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:42:59 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:43:30 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:43:55 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:44:25 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:45:30 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:45:35 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:45:55 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:46:30 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:46:55 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:47:36 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:47:46 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:47:56 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:48:25 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:48:55 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:49:29 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:49:55 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
sept. 25 16:50:30 omano-nvme kernel: r8169 0000:05:00.0 enp5s0: rtl_rxtx_empty_cond == 0 (loop: 42, delay: 100).
Here is journalctl:
journalctl -u NetworkManager.service -f
-- Logs begin at Sun 2020-09-13 02:58:12 CEST. --
sept. 25 16:46:07 omano-nvme NetworkManager[552]: <warn> [1601045167.1097] dhcp4 (enp5s0): request timed out
sept. 25 16:46:07 omano-nvme NetworkManager[552]: <warn> [1601045167.1111] device (enp5s0): Activation: failed for connection 'Connexion Ethernet'
sept. 25 16:46:52 omano-nvme NetworkManager[552]: <warn> [1601045212.0987] dhcp4 (enp5s0): request timed out
sept. 25 16:46:52 omano-nvme NetworkManager[552]: <warn> [1601045212.0999] device (enp5s0): Activation: failed for connection 'Connexion Ethernet'
sept. 25 16:48:09 omano-nvme NetworkManager[552]: <warn> [1601045289.1080] dhcp4 (enp5s0): request timed out
sept. 25 16:48:09 omano-nvme NetworkManager[552]: <warn> [1601045289.1092] device (enp5s0): Activation: failed for connection 'Connexion Ethernet'
sept. 25 16:48:57 omano-nvme NetworkManager[552]: <warn> [1601045337.1083] dhcp4 (enp5s0): request timed out
sept. 25 16:48:57 omano-nvme NetworkManager[552]: <warn> [1601045337.1094] device (enp5s0): Activation: failed for connection 'Connexion Ethernet'
sept. 25 16:49:42 omano-nvme NetworkManager[552]: <warn> [1601045382.1064] dhcp4 (enp5s0): request timed out
sept. 25 16:49:42 omano-nvme NetworkManager[552]: <warn> [1601045382.1079] device (enp5s0): Activation: failed for connection 'Connexion Ethernet'
For now I can have proper connection by forcing the DNS server. That doesn’t make sense to me but OK, problem kinda fixed I guess.
// EDIT: it actually was just pure ‘luck’ or ‘mystery’, in the end nothing was fixed.
new magical fix/band-aid/nothing: I enabled IPV6 in my router (but still disabled in NetworkManager) as a last resort for the sake of trying, and it seems to work… I did that because an old TimeShift backup also has the issue now so it seems to be external to Manjaro now… 
Spoiler from future still not fixed it was just another fluke