To avoid excessive consumption of UDP port that seems to be detected
and blocked by SIAF upstream firewall.

Only loopback relay is permitted, so the only TURN Topology allowed
is:

Forward:
Client 1 <--TURN--> (udp/88) Server <--RTP--> Server (udp/88) <--TURN--> Client 2

Topologies like:

Client 1 <--TURN--> (udp/88) Server (udp/>10000) <--RTP--> Client 2

Are not allowed.

Configure a TURN/STUN server, where user can be authenticated using a
REST-API with a shared secret (for example Matrix-Synapse users).