The new beta version of Synology ChatPlus/Meet is here.
The GUI will not accept a domain such as "turn.example.com", even if the DNS resolves to the right IP. It has to be the IP – such as 199.198.197.196 (or some such). If you set it up in a homelab and forwarded the ports in the router, the IP you need is the WAN IP on your router.
Source: r/synology · by /u/The-Lone-Paddler