It sounds like the router has failed. I rarely fuss with old routers but always try the full reset then set it up for a test run. If it fails I swap in another router and re-test.

If I suspect a PC, I always carry in my phone and laptop to see if they work. Owners can take their phone or laptop to a free hotspot to see if it works out there.

In parting, always try FORGETTING all networks in phones and PCs.
Bob