NAT on HD2 bonding 2 SIMs, customer VoIP doesn't work due to double natting

Hi everyone hoping for some help or feedback. I have a customer using a HD2 which has 2 LTE SIM cards bonded via FusionHub. The HD2 is and the customer router connected to it is In FusionHub, I turned on Port Forwarding for all TCP and all UDP and set to forward it to FusionHub is set as IP Forwarding and has the option on about SpeedFusion peers access internal network checked off.

The customer is saying their VoIP phones won’t work because of double natting and want FusionHub’s IP assigned to their firewall which I know is not possible. They are asking me to create a static route to the firewall because call traffic isn’t being routed properly. I’ve never done that before so not even sure how or if that would solve it.

If all outbound traffic is set to go through PepVPN and inbound on FusionHub is set to forward to their firewall’s IP, then shouldn’t VoIP just work? Anything else I should ask them? Thanks!