If it can help you, you should know that they test everything over a VPN that doesn’t support UDP. So if your app makes use of UDP you’ll need a fallback method.
UDP is fine. We have apps on the App Store that use UDP. This is likely a NAT related issue. That is not going to be an issue just for App Store review, but also for tons of other networks out there. As of lately, carrier-grade NAT deployments are very common.
It's usually impossible to establish a two-way UDP
"connection" between two peers that are both behind NAT.
This varies depending on the type of NAT. If just one peer is behind NAT you can typically establish the "connection" using NAT hole punching. If both peers are behind NAT you may need to proxy the UDP packets using an intermediary server.
There are protocols for hole punching, such as ICE (https://tools.ietf.org/html/rfc8445), but you can also roll your own - if you control all endpoints.
"Connection" in quotation marks because UDP is a connection-less protocol, but the NAT port mappings that are established are sometimes referred to as a connection.
