New devices do not connect

Check what type of wireless security encryption the Router is set at for the security key.
It will be either WEP, WPA, or WPA2.

WEP is the oldest of the three and therefore almost any wireless equipment is compatible with WEP. However, if the router wireless encryption is set to WPA or WPA2 some wireless devices cannot understand WPA or WPA2 encryption which means those devices won't connect.

The answer in that case is to set the Router to use the type of encryption which all wireless devices can understand. That usually means setting the router to use WEP encryption even though it's not as secure as the later standards.

To check or modify the router's wireless settings you'll need to open a web-browser, type http://192.168.0.1 into the address-bar & press <Enter>

To add on to what pip22 said, one other thing to check in the router settings would be to make sure the "Maximum DHCP Users" is set to something higher than 4. I believe by default it should be set to 50 or so but if somehow that got changed, then your router could just not be assigning more than the 4 IP addresses of the 4 computers already using the network.

Thanks for your input, guys. I reset the router back to default. Checked to make sure the settings you mentioned were right. Had a few kinks but eventually got it up and running.