SSL traffic fails to respond from WAN interface

Hi all

I have just provisioned a new WAN interface (LTE router) as a failover WAN at a site with latest UTM.

Any traffic from this WAN that hits an SSL endpoint on UTM or LAN (incl. WebAdmin, SSL VPN, SSH & other HTTPS servers) simply times out.

Any traffic hitting a HTTP service works just fine. Settings all the same. 

If I replace the LTE WAN with the fiber WAN, it works perfectly fine.

Netcat shows successful TCP handshake and connection, but no data is returned.

EDIT: just setup a netcat listen (nc -vvv -lkt 6666) on an internal LAN server, NATed the ports, tested 2-way "telnet" communications - works perfectly)

Nothing helpful shown in firewall logs.

Tried using valid HTTPS certificates - Still no reponse

Has me completely stumped.

Have the same LTE device and service at another site with an XG box and it works perfectly fine. (LTE router settings are exactly the same at both sites) 

Anyone have a ideas?

Thank you