Hello Aarhus!
Thanks for jumping in and teaching this noob 
I’ve referenced again with your examples, this is my modified home-yury-Jnternety-Documents.mount :
[Unit]
Description=JnternetyDocuments Mount
[Mount]
What=//fritz.box/jnternety.nas/Documents
Where=/home/yury/Jnternety/Documents
Type=cifs
Options=_netdev,iocharset=utf8,rw,file_mode=0777,dir_mode=0777,credentials=/etc/samba/credentials/jnternety
TimeoutSec=30
[Install]
WantedBy=multi-user.target
after running sudo systemctl daemon-reload and systemctl start home-yury-Jnternety-Documents.mount, I’m getting this ls -l output
total 3552
-rwxrwxrwx 1 root root 2751518 4. Jul 16:41
drwxrwxrwx 2 root root 0 15. Jul 15:39
-rwxrwxrwx 1 root root 158208 4. Jul 22:22
drwxrwxrwx 2 root root 0 26. Jun 16:18
drwxrwxrwx 2 root root 0 25. Jun 23:42
-rwxrwxrwx 1 root root 483931 25. Jun 10:33
-rwxrwxrwx 1 root root 217748 25. Jun 08:38
drwxrwxrwx 2 root root 0 30. Aug 12:36
drwxrwxrwx 2 root root 0 25. Jun 23:05
drwxrwxrwx 2 root root 0 26. Jun 15:24
My drive at the FritzBox router is formatted ext3, so I followed your point 5. Check permissions and have run
sudo chmod ugo+rw /home/yury/Jnternety/Documents -R
Which didn’t change ls -l output at all and I still can’t open files NEITHER as user NOR as root…