IMO, there doesn’t appear to be much to worry about.
A private IP address is on your local network so it is probably assigned by your router. You would normally check the router’s DHCP / NAT assignment table to confirm this. Although you said " I can’t access my router settings", you have “changed my the wifi’s password” which normally requires access to the router settings. So you could turn off WiFi access at the router and see if the devices disappear in Glasswire.
You can see which device is the router because it should be the default gateway for Internet access. So you can get its IP address from the network connection properties on your computer or phone.
Do you have “guest” access enabled at the router? That can mean that no password is required so any device can connect.
Are you on a wireless ISP network where other subscribers systems may be visible to you? See the following topic for an example of this but notice that this user didn’t have a router so it probably won’t be the same problem as yours: