We need to know specifics of your setup to assess properly and make appropriate recommendations.
For your modem/router, what is the Make and Model?
For your computer, could you share the results of the following command?
inxi -SMN -xxx
Also, on your router, along with anything else that might help, you need to confirm these two settings are “sane”.
[1] Disabling Guests
Ensure that you find the option screen for Guests and turn that option off to block “guests”, similar to what is shown below:
[2] Allowing ONLY Registered Devices
Maybe … Choosing the option to only allow recognized listed devices. Again, look for the screen where you can control that “mode”, similar to the below:
