One of my devices can't see or be seen by the other device (could the app recognize it as a separate network?)

[IckoKirov]

ok, so i have several devices - phones, tablet, laptops and all of them can see each other over the wifi network.
but I have a desktop PC that is connected via ethernet that can't see the other devices, and it cannot be seen by them as well.

the specifics are kind of messy tho, my network is as follows:

• a regular main router (singular router with the 1-in 4-out ethernet ports), but with wifi turned off.
• then there is a wifi mesh system that is connected to this router, it automatically adjusted itself to receive the internet connection from the main router and spread it over wifi

so, naturally my phones and tablets are connected to the wifi mesh routers, but the PC in question is connected to the main router with ethernet cable.
now my guess is that the app don't recognize it as the same network or something, and that is why this PC is "invisible". But the thing is, everything else that requires local network is working fine - steam is able to stream from and to the PC; Jellyfin server that is on this PC is visible from all other devices and so on.

so is my suspicion correct? if so can this be fixed? in another way of just reworking my network altogether or connecting the pc to the mesh network (since i would need to buy a usb/pce wifi for it)?

The PC in using Linux Mint 21.3, and the 53317 port was already allowed in the firewall settings (probably when i installed LocalSend)

P.S. I saw a similar discussion but it was way too broad, and mine seems to be kind of a corner case, so i decided to start a new discussion.