I need a systems engineer to resolve a firewall issue

I need a systems engineer to resolve a problem that we have with our hosted PBX which is not working due to a firewall issue.

My network consists of the following:

SERVER
Dell PowerEdge T330
OS: Windows Server 2016 Standard running Hyper-V

FIREWALL
Dell SOHO (SONICWALL) 12800
Interface ‘X1’ is our failover ADSL internet connection
Interface ‘X2’ is currently being used as our primary 4G LTE internet connection which is provisioned for outbound only.

Note: A basic failover to 'X1' from 'X2' is setup on the Firewall.

MODEMS
--------------
DrayTek Vigor130 (ADSL)
Running in bridge mode so no access to user interface or IP address.
Ethernet connectivity/authentication via the SONICWALL Firewall WAN X1 interface.

Cradlepoint ARC CBA850 (4G LTE) Modem
(fitted with MC400LP6 Dual SIM Cat 6 Modem)
Running in routed mode.
Ethernet connectivity/authentication via Firewall WAN X2 interface.

TELEPHONY
Gamma Horizon hosted PBX
(2 x handsets & 2 x PC softphone clients)

Firewall rules (as provided by Gamma) are configured to allow traffic to and from the hosted PBX over our ADSL line.

PROBLEM:
We are unable to receive or make calls from the handsets and softphones unless we either disconnect our 4G connection or swap over the firewall failover interface ordering making X1 (ADSL) our primary connection. We don't want to do this because our ADSL internet speed is 15 times slower than our 4G connection.