To be universally compatible, any modem/gateway UPS product would need to provide AC power. That isn’t going to make for an inexpensive device. I have a couple of APC units that use a lithium ion battery pack, and while smaller than the typical unit that uses a sealed lead-acid battery, they still aren’t that small (about the size of a hardback book), and even on sale back in 2017, it cost around $40.
It also penalizes those who already have a power backup solution in place (such as the UPS I and many others have, or the standby generator an older friend of mine has).