Almost 10 days of holiday internet: The "RPi with Android phone as LTE modem over USB tethering" is surprisingly stable. Just had to buy the second 6GB data volume extension (so we used up 12GB by now).
The downside of using USB tethering is that it has to be manually activated on the phone after the host USB port went down (reboot or such), though maybe that could be automated with something like Tasker? On the upside, the phone doesn't run out of charge, and I just need a single power supply.
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