Wifi status 'Connected, no internet'. Can access all internet resources I've tested Related question

I've put a new router into the office to run our 'guest' wireless network. It's a FRITZ!box 3490.

Android devices connected via both 2.4 and 5G are showing as 'connected, no internet' except that they are all able to access the internet.


What mechanism / method does android use to determine 'internet access'


Everything I've tested (ICMP/HTTP(S)/DNS/Outlook [SMTP]/Sharepoint/YouTube/Plex/Spotify/Various Google Services) all work as expected so it isn't really a problem but I've had a steady stream of people come over and tell me that they can't get on the network (even though they can if they'd just try it).

From the above linked question it is mentioned that the android device calls home to google to confirm 'internet access'. This must be being blocked but I can't see what traffic is blocked / not routing to cause this behaviour.

Sadly the new routers firewall interface leaves much to be desired so it's not clear whats being restricted, even after I've set everything to 'Unrestricted' in the router firewall interface.


Is there a list of google systems/services/addresses that need to be available to get this working?


EDIT.
From the linked question a hidden comment suggests that the android devices are attempting to connect to https://google.com/generate_204

Whilst connected to a network that is ok i get the following from a cURL against that page:

&gt; curl http://google.com/generate_204
HTTP/1.1 204 No Content
Content-Length: 0
Date: Mon, 10 Sep 2018 11:21:24 GMT


Whilst connected to the network with the problem

&gt; curl http://google.com/generate_204


There is no output on the second curl. This must be the root of the issue, now to work out where/why my router is blocking this.
Ashamed to say rebooting the router fixed it.