Assuming you mean Android, FYI syncthing for android is discontinued, so you might want to look into other options.

https://github.com/syncthing/syncthing-android?tab=readme-ov-file#discontinued

I really like Zoraxy. Similar to NPM but it’s its own thing and I like it a lot more

I know how to use raw nginx/Caddy/traefik to do it, but I find the WebUI and all the extra features Zoraxy has to be very convenient and easy to use.

AKA, split DNS. Doing it this way is a bit cleaner than hairpin NAT as mentioned in other comments, but both options work fine in a home network.