This situation is why we need a test setup specifically for the client VPN, if that's even possible.
I don't think using the 802.1x test off your wireless would necessarily be too helpful here, since that'd be testing off your APs instead of the fw. Did you copy your 802.1x config over to the new NPS server, with your APs as valid clients?
If you're using that test button and not getting a valid test when you should, you probably should check to make sure that packets are actually leaving your device and being received by your NPS server. You can do a pcap off your Meraki device, and then check your NPS logs on your server.