that was just a guess it was wireguard. https://search.brave.com/search?q=wireguard+udp&source=desktop&conversation=6aea02d872b7fd5286fdd0&summary=1
it is possible that they were not blocking UDP and it generally uses random high ports.
wireguard is awesome btw. i use it all the time.
Discussion
btw, HTTP/3 also uses UDP, but not many servers support it yet. not sure how good browser support is either. but it's awesome, like, so nostr uses http/1 or http/2 websockets, the equivalent with http/3/QUIC is "WebTransport" which is far better - not only is it over UDP, the sockets multiplex so where previously clients might open 5 separate websocket connections, each of them taking like 200ms of upgrade negotiation, with WebTransport it's one connection and i could be misremembering but the upgrade is not a round trip, you just fire off the open and then follow straight up with your requests and each goes on a separate part of the multiplexed connection.
probably is a couple years more before WebTransport and HTTP/3 are in common use but obviously this ISP will have to update their network security