Think more like "network reset" than "factory reset" (do you want to have to start all over just for this)?
Here's how to do a network reset:
My guess is that your hub is holding on to its old IP address from your old router, so this might help--as could a reboot, but without access to the regular hub interface, this is something else you can try (and it does a reboot too after it clears any network settings you may have had configured).
Also, unless a static address was setup on the Hubitat hub, couldn’t a new IP address be setup on the new network provider’s modem for the Hubitat Hub’s MAC address?
The procedure highlighted above by @bertabcd1234 will reset a static address I think…