Multiple devices connected to a single network, but only myself and the router can be detected

It should be connected to the same network, but I'm having trouble finding devices other than myself and my router.

I use Buffalo's WHR-G301N router at home.
If you use an app called LanScan on your Mac or Fing on your iPhone,
You can view a list of devices connected to your network.

However, even if you check with those apps,
Only myself and my router appear on the list.

Example
Search on terminal A: Only terminal A and router can be detected
Search on B terminal: Only B terminal and router can be detected
前提As a precondition, both terminal A and terminal B are connected to the same network.
->Connecting to the same network, only myself and the router can be detected

Is there something wrong with the network?
If anyone understands, please let me know.

Because of this problem, when accessing a local website or API in a Mac from a real smartphone,
I tether my smartphone every time, hang my PC on it, and check it out.
(With this method, you can access it by specifying the IP you configured on your Mac.)

Thank you for your cooperation.