BTW for anyone interested - tschat-1.teamspeak.com is indeed federating. It’s really just missing delegation, so when you configure the required SRV record in the DNS server your synapse uses it’s already working.
In unbound it’s something like this:
local-zone: tschat-1.teamspeak.com typetransparent
local-data: "_matrix._tcp.tschat-1.teamspeak.com. 300 IN SRV 1337 1337 443 tschat-1.teamspeak.com."
