Strange LocalTuya issue with a single device

I have two inline switch devices - you know the sort: couple of inches long, small black button, used to be able to run Tasmota on them before Tuya cracked down on it - which I use to control lamps.

I’ve set up LocalTuya and it seems to work fine in the app to control both devices.

One of them works fine in HA and is always available online. The other one just goes unavailable after about 20 seconds. If I turn it back on in the Tuya app it becomes available in HA again for about 20 seconds, then goes back offline.

What’s weirder is that the Tuya app can see, even when it doesn’t show up in the router. It has a fixed DHCP address allocation, so always comes up back up inthe same IP. When it disconnects from HA, it disappears in the router. When I activate it in the Tuya app it reappears in its correct IP in the router.

How is this possible? If it’s not going through the router and being allocated an IP address then how can the Tuya app see it externally??