Hi @mocelet Yes, the Smartthings hub is on the same VLAN. And sorry, the only other Meross devices are the Smart plugs. I did use account integration rather than Matter - I tried adding the Meross hub as a Matter device to Smartthings but got an error code (can’t remember what it was just now) that implied that I needed to add the Meross hub through the integration rather than directly.
I have turned the hub on and off again several times now. Each time I turn it on, I get asked to add the MSH450 hub to Smartthings and every time the process stops with the error code 39-100. Based on what @mocelet said about the plugs being connected through the cloud, it seems that the issue is that I can’t add the Meross hub to Smartthings so will start a new thread on that topic
Do you have any Matter device installed? Maybe it’s not a device specific problem and just something blocking Matter traffic due to your network setup.
If your smart plugs are Matter compatible (like the MSS315) you could try adding one via Matter. Plus, you might be missing out on the power reporting features, available via Matter with recent firmware updates but not via cloud.
Forgetting about Matter, another question is if the sensors are compatible with SmartThings using the cloud integration, then they should appear as a device in ST assuming they are installed in the same Meross account linked to ST. You could try asking Meross support if the sensors are compatible with SmartThings using the account integration, maybe they aren’t.
In Tapo at least you have both options, the account integration and Matter. They’re independent and you can even use both at the same time although that usually means having duplicated devices in SmartThings. I would expect Meross to follow the same strategy.
Yes, I have a Zigbee smart bulb connected directly to the ST hub and used to have the Matter Meross plugs connected directly also. Good suggestion about asking Meross support re integration - will try that
What happened then so you had to remove the integration?
I deleted them before I added the hub as I didn’t want them appearing twice in ST
Last try: remove the Meross hub from the Meross app, factory reset it, add it back and try again to add it via Matter to the SmartThings hub.
If you only have that single one temperature sensor that you want to use in SmartThings, get rid of both the Meross hub and the sensor and get one that you can connect directly to the SmartThings hub.
Factory resetting seems to have done the trick 
. I can now see the sensor inside ST - what a performance that was! Just wondering now if I add the Meross smart plugs through the hub or through the integration. One for tomorrow …
The goal should be to have one hub - the SmartThings hub. If you add them through the cloud-to-cloud integration, here won’t be any local automations.