Why not use fake-hwclock in combination with NTP? That's what I use on my SBCs without RTC:
$ echo "NTP=pool.ntp.org" | sudo tee -a /etc/systemd/timesyncd.conf
$ sudo timedatectl set NTP true
$ sudo systemctl enable fake-hwclock fake-hwclock-save.timer
$ sudo systemctl start fake-hwclock-save.timer
Edit: Nevermind, I just re-read the OP about lack of internet. Is there a util in the DVB utils that exposes the network, and perhaps a cron job that re-syncs the clock, that would work?