Not a redundancy option, but I also had my routing setup go in to a bad state while traveling which was a hassle.

I solved this by setting up nightly reboots while away. Both my routing PC and modem are rebooted by smart switches (zwave/ZigBee) controlled by home assistant. Which means they’ll operate without a working network. The routing PC is set to shutdown one minute before the smart switch turns off, and set to boot automatically when power is restored (smart switch turns back on). Which avoids any issues with hanging on a reboot.

Perhaps there are local classifieds or marketplaces you could use? For example, one of the local news services in my area runs one that’s well known and active.