I know you'd prefer things to be working but honestly I'm slightly relieved. Some of the magic has gone away.
The fastest test would be to throw a different router in place of your current router. A different brand/model would be best. If the new one has the same problem then you get to complain.
Next fastest (but better if you don't have another router) would be to update firmware (check readme for VPN or relevant changes), reset factory defaults, then configure as needed. See how it behaves. Local routers can play with VPNs though problems are usually more black & white. Like, "it doesn't work".
Yes, the ISP could affect it. Some YokelNet could decide to charge for business accounts and institute traffic shaping to recognize and shut it down. Would depend on the provider and the VPN. Back in the day AT&T YokelCable blocked VPNs for other than business accounts. Bastages. I'm sure some still do this though my few VPN customers have business acounts so I haven't seen it in many moons.
That might not be a bad idea. Check your ISP's terms for anything having to do with VPN's. It would be a coincidence that some policy kicked in right after your power outage, but troubleshooting is filled with coincidence.