Smartphone Radar Not Working After Router Change.

I used to have an AirPort Time Machine 4 as a router and the SmartPhone Radar worked well with it from day one.
I just changed the router for an AirPort Extreme 6 and the plugin stopped to work properly.

The problem seems to be that the AirPort Extreme is not responding, so the plugin "times out".
Nothing else has changed other that the router.
The plugin configuration seems to be correct.
The router configuration is the same as before.

Am I missing something?
Is the AirPort Extreme 6 (hardware/software) not compatible with the plugin?

I'm running:
Airport Extreme Software v7.7.9
iMac 2012 with macOS v10.14.1
Indigo v7.2
SmartPhone Radar v1.1.3

Any help will be appreciated.
Thanks in advance.