for some reason one of my phones won't connect to my wifi but the other one will

Restart your router by removing all power for a few seconds.

The first thing I would do is to unplug my router from my wall socket and wait a few minutes and plug it in again to get full power and wait until all the needed lights are turned on the router again. Or if it does not solved your issue you are having then the next option is to call your router customer service 800 phone number and asked for technical help. Its much faster this way to talk to someone who made the router instead of waiting for a answer from someone on this Apple discussion site.