While the cronjob in the first message of this thread seems to work well, I have an improvement.
The problem is, if I exceptionally schedule an alarm earlier in the Clocks settings, the sound/vibra may be still turned off. So it would be better that instead of sound/vibra the modem would be turnded off. To achieve that I have changed the cronjob /etc/cron.d/no-disturb:
XDG_RUNTIME_DIR=/run/user/1000
# disable modem at 22:00
0 22 * * * root mmcli -m any --disable
# set sound (80% volume) and vibra for next day
0 0 * * * purism amixer -q set Master 80\% && gsettings set org.sigxcpu.feedbackd profile full
# enable modem at 6:30
30 6 * * * root mmcli -m any --enable