AC1900 connects to my centurylink router with no problem but then eventually freezes.

what extender did you buy? AC1900 is a speed. 

When you're only dealing with a 2-3 meg connection, I'd highly recommend using netgears fastlane. 

A standard dual/single band extender cuts throughput in half. This happens because the signal has to go router----extender and then extender---devices and the chip that routes it, can't do both at once. 

By using fastlane, it dedicates one of the bands for just router----extender communicaiton. I don't usually recommend fastlane as 2.4hgz is a slow network and can be a bottleneck. But when you're only running at 2-3meg, thats the bottleneck. 

So try logging into the extender and set it up using the fastlane method. 

Also, extenders won't eliminate buffering if devices are buffering because of your slow connection speed.