Sorry for not being clear. Let me elaborate:
I am testing wireguard from my mac. it access a high UDP port on my server. It works perfectly with my home ISP.
Now I plug my travel AR300M router in front of it and use it as an access point, connect the mac to it. All internet connections work fine. But wireguard from my mac does not work anymore. So I suspect that the AR300M is blocking outgoing UDP packets.