Mobile WiFI no internet

In the last few days all our mobile devices have been reporting 'no internet access' on our LAN WiFi across Android and Apple devices.

Laptops and TVs are fine.

Initially I thought it might be connectivity-validation hostnames being blocked but I couldn't see any in the logs.

We use dns over tls with our asus router.  We've had this setup for a while with no problems.

The mobile wifi issues have been getting increasingly worse.  Any ideas community what the cause and fix might be ?