iPhone 6 Wifi problem on only one router

I have an iPhone 6 on iOS 11.2.6. I have an airport express router (version 7.6.9).

The phone works on other routers, at work for instance. And all other devices in the house connect to the router without problems, but my iPhone 6 has constant issues. However, it will for instance, load the Facebook news feed, but when I click a link, it won't load that. Safari basically won't load at all and trying to set up devices over wifi with the phone doesn't work at all.

I've tried a number of things on the phone, like resetting network settings and even completely wiping the phone and trying it as new.

Anyone else have a airport express with issues like this? It has worked fine in the past, and my old iPhone 4s still connects to it without problem. It's just the 6 that is struggling.

Any help will be greatly appreciated, thanks!