There is no problem to run Syncthing behind DNAT with directly specified DNAT server’s address/port on peer devices.
But i did not find a way to specify this address/port in Syncting settings to be passed to the announce server instead of (or in addition to) internal address of machine running Synctihing.
Use the same port both internally and on the NAT gateway and it’ll be correctly announced. There is no other workaround currently. (Apart from UPnP which sets this up automatically.)