re: UPS

It's worth it. The Modem and ONT pull almost no wattage at all. I recycled a Cyberpower 500VA? unit to stick on each device (my modem was upstairs, the ONT was in the garage.) The ONT measured at over a week of expected uptime, and the modem four or five days, but that was also with a 24 port switch and a 5 port PPoE switch plugged into it. Oh, and a NAS.

You are an absolute fool if you rely on tethering off of your phone during a power outage.

1) 5G/LTE have shorter range than 3G, so if the power is out in your hood, and you're not near an interstate, then the LTE tower a mile from your house is probably down too. This was certainly the case during the Houston deep freeze. People couldn't send text messages until I told them to disable LTE. Then, that was almost all they could do, except refresh centerpointenergy.com and wait for it to refresh the page for 30 seconds.

2) Many major carriers distinguish between mobile data use and hotspot use, and charge overages much faster on hotspot use.

3) What do you think your neighbors are going to try doing? Tethering.

Buy the cheapest Cyberpower UPS for the ONT/Modem, and get another one for your Wifi router (if you have one.)