Port forwarding not working on BT Hybrid Connect?

[techsupportplease]

Hi,

I have got BT Full Fibre with a Smart Hub 2. I have also got the BT Hybrid Connect 4G modem to fail over to if the fibre line goes down. The 4G modem is connected to the Smart Hub 2 using a LAN port on the Smart Hub 2.

The Hybrid connect works fine and kicks in if the main fibre line fails.

I have a server on my network and I use port forwarding on the Smart Hub 2 to access it externally. However when the network fails over to the 4G hybrid I can no longer access the server. I know that the external IP address changes when it switches to 4G but even knowing the new external IP address and trying to access the server from this new IP address, I cannot.

I thought it might be because the 4G modem might be changing the internal IP address of the server but it doesn't seem to as the IP address remains the same. It's the Smart Hub 2 that still acts as the main router and remains in control of the IP address allocation, I think.

So can anyone think of a reason why the port forwarding might not work when the Hybrid is in use and what I could do to fix it? There doesn't seem to be any way of getting on to the hybrid modem directly to see its settings and so everything seems to have to be managed through the Smart Hub 2 interface.

Any thoughts would be gratefully received.

[Keith_Beddoe]

@techsupportplease 

Its more likely that the 4G modem does not have its own unique public IP address, and is in fact connected to a shared gateway, where the same IPv4  address is used to supply a connection to multiple users.

This is called CGNAT and is in common use on mobile networks, as its a very efficient way of using a single IPv4 address.

You  would not be able to establish an incoming connection under these conditions.

https://en.wikipedia.org/wiki/Carrier-grade_NAT

[licquorice]

You can check by comparing your WAN IP address and what 'whatsmyip.com' says.

[techsupportplease]

Thanks for this Keith, much appreciated.

I might try using an IPv6 address instead and see whether the non scarcity issue with IPv6 is recognised and CGNAT won't apply

[techsupportplease]

Thanks I will try IPv6 and see whether that works instead

[DavidM]

Hi @techsupportplease and welcome to our community.

I'd be very interested to know if you get the sorted so please keep us updated on your progress. I'm sure the other members on the community would be interested as well.

Cheers

David