the modem is disabled after the first reboot
Did you turn off mobile data with the toggle in the top drawer? If so, you have to enable it for now, because the toggle currently turns the entire modem off. (Or you can try manually enabling the modem with mmcli as a workaround.) See Modem starts completely disabled if mobile data is disabled (regression from ModemManager port) (#133) · Issues · Plasma / Plasma Mobile · GitLab.
This was fixed a few hours ago for Plasma 5.24.1, which will be released upstream in the next couple days (tomorrow Tuesday 2022-02-15 if it goes according to plan) and then hopefully trickle down to Manjaro unstable → testing → stable (→ new Beta, for those who prefer reinstalling).