DNS SRV is only partially supported on Asterisk/Freepbx using the CHAN_SIP protocol and while it recognises the first return destination on the DNS SRV records it doesnt handle failover very well.
We recommend using our manual settings with primary and failover servers both configured
https://www.irishvoip.com/index.php/knowledgebase/41/How-to-use-a-SIP-Trunk.html