@kuba When this happens, I'd run packet capture on the raspi to see if it truly receives those packets. I would also maybe try to send packets to all IPv4 IPs and see which ones end up there. If not, then this is even more broken than your ISP intimates. If yes, you get to see what IP ranges get misrouted.

I'd also run a long-term packet capture on that raspi (possibly filtering high-volume streams and possibly periodically deleting old packets as long as the problem hasn't manifested), and wait until the setup breaks again. We would then see what was sent by the raspi just before the setup breaks.

(I don't see a reason for a ISP-provided router/access point/... to talk any routing protocol on the customer-facing interfaces, and frankly little reason for it to talk a routing protocol _at all_, so that router is doing something weird anyway that arguably is a bug. Unfortunately, it's not very surprising that some sort of weird or incorrect behaviour is going on there. What I'd hope to see from the packet captures is (a) what triggers the misbehaviour, so you can in the worst case filter that out (b) possibly figure out whether that's a vulnerability that e.g. would allow someone on the outside to do the same if e.g. you have a connection open to them.)