I have a very similar experience with my Shelly Plugs S. One of them is not available every morning and you can see its lights blinking trying to reconnect. On the other hand, another Plug S is working perfectly since installed (it's connected to a different AP).
I have an Asus Mesh network and I think that's the issue, the mesh configuration or something. I have a similar experience with my Shelly 2.5s, random disconnections.
To recover the connection quickly, instead of restarting the router, I just change the Wifi 2.4 GHz channel and the Plug S and other Shelly devices reconnect immediately but it's very annoying to go through this process almost daily.