WebRTC can create a NAT Mapping via STUN (UDP). https://webrtcforthecurious.com/docs/03-connecting/#nat-mapp... that is pretty powerful compared to Client/Server HTTP.

Dependent on the firewall, but most I have seen allow NAT mapping with different behaviors.

To my knowledge, most regular web servers either cannot speak DTLS at all or only have limited support for it, but all modern browsers work with it just fine for WebRTC.