Phones not connecting to PBX over failover ISP

Thank-you, when I plug a phone into my standby ISP modem, the phone gets an ip but does not connect to the pbx (the pbx is in the cloud). I was lead to this behavior when I failed over our entire network to the standby modem and all was working except the phones. All of our phones are configured to connect remotely to the PBX. My question is: Is there a config option in FreePBX I need to set to allow the phones to connect to the PBX from a different ISP? - The modem’s firewall allows sip and rtp traffic btw.

The phones are Linksys IP Phone SPA962


Are you specifying which path to take to send SIP & RTP?
Do you have to maybe change that once you flip to the other provier?
Look at your firewall logs, I’m sure you will have a better picture of what is happening.