Ensuring Tele2 connectivity via 4G requires running fiber first?!

This is probably one of the stupidest things I've ever heard…

The parents' internet connection (Tele2 FiberLAN 500/500) went out shortly after lunch on March 6th, and has been out ever since. Media box is acting strange; At first it seemed like the connection/lost connection every few seconds (but it was not possible to access the internet from the computer, even for a few seconds), then the PWR and LAN2 were resolved for only a few days, and last time I checked all the lights were flashing yellow Except for PWR and USB, which are completely turned off.

They recently upgraded to 500/500 and thus have Tele2 Guaranteed communication, that is, you should be able to browse via the router's 4G modem, which is automatically used when fiber is cut – but they were away during the move, so it was installed only yesterday. However, I installed the same router before it crashed, and it was working fine and at the right speed for a few days before it crashed.

Their connection still doesn't work, and according to Tele2 support, the problem is that the fiber is down — the fiber has to work first, and Then when it gets low you can switch to 4G. So, their internet and TV were out for 10 days (while they were away), and the outage will continue (now that they're home) until at least Tuesday, when the technician will check the whole thing out.

I suggested plugging the dongle into a computer instead, which Tele2 says should work (for example, if there's a power outage), but according to support, it shouldn't work either until fiber is up and running…
However, it is not a good solution as it does not solve TV problems or anything related to phones and tablets.

Does anyone know if this is true and if there is a way around it?
I don't know exactly what they mean, if the 4G thing has to be activated once – in that case, maybe you can take my 4G router + modem (which has a Bahnhof) and run it that way?

Instead, there must always be transmission from the 4G fiber network while the router is powered, otherwise it will be finished.

Instead, the support is at fault and a workaround can be found.