Yes, the Problem is only about things behind the switch.
Thans to the Link from @frollic Local Lan now see each other now the VPN is still not possible to do that.
(Local Lan 192.168.100.1-255 and VPN 192.168.150.1-255)
My VPN sees devices connected to the router but not connected to the switch.
Ping from Windows on wireguard VPN client give me a timeout.
Check the local firewall on your NAS... make sure it will accept connections from other subnets (this may be prohibited by default).
If that doesn't fix the problem, double check to make sure you can reach the NAS from a host that is directly connected to the main router... if that isn't working, something else is wrong and we need to dig deeper.
Just to clarify, "this device" is the ubuntu machine? When connected via the VPN, is being done from inside or outside of the network?
This doesn't make it appear to be outside, if the device is connected locally to the network.
Test using a phone (cellular connection) or from an actual remote network.
With all of the test scenarios, all of the test appear to involve that switch. Please try taking one of the devices that is currently connected to the switch and connecting it to the main router intsead...
Specifically, let's say you have:
device A (your phone or you ubuntu machine) connecting to your network from a remote location over Wireguard
Device B is currently connected to the switch.
--> Device A cannot reach device B.
now, take device B and connect it directly to the main router.
--> can device A reach device B??