WAN Failover to 5G

Hey folks..

I've got the NBK752 configured as ethernet (from my ISP)  as primary will failover to EE (UK) 5G if ethernet fails...

The issue I have is the failover doesn't appear to be working:-

Router shows 4G connection status as connected with WAN IP etc but there is no onwards connectivity to any devices on the network. If I try to a n/w speed check on the router itself then even it says its not connected. 

Once the router is rebooted works all perfectly - but I'd kinda  hoped it was fully automatic and as a travel a lot and really didn't want any manual intervention. Is there a setting somewhere I've gone wrong?

Thanks in advance

Simon