Setting up a backup internet connection is trivial nowadays. All you need is a Ethernet to USB adapter and if this Ethernet adapter1 happens to not be USB-C, a USB-A to USB-C adapter.
Now all you need to do is take your smartphone2 and enable “Ethernet tethering” in the hotspot settings and connect the USB to Ethernet adapter to the WAN port of your home router.
Voilà, you have a cheap backup internet connection you hopefully never need. To be fair, it’s not fully automatic failover and it really depends on your data plan, but it’s much cheaper than actually buying an additional internet connection for your apartment, which would probably fail along with the main one anyway.