iPad 2 freezes constantly on the same wifi network an iPhone 6 works smoothly
If one device works fine on the network with a network-bound app and the other does not then it is not likely the fault of the network. And as you have replaced the WiFi router on your network and it still has problems you need to look at the iPad.

The first thing I would do is reset the iPad's network settings like this:

Settings &gt; general &gt; Reset &gt; reset network settings


If that does not fix the issue I would back up the iPad to your Mac/PC via iTunes and go back to:

Settings &gt; general &gt; Reset &gt; Erase All Content and Settings


And wipe the iPad. Once you do that restore it from the backup you just made and try again.