pb.forumcustomdomain.com has no SRV records.
Question and response
Question
dig n.sni.global.fastly.net. SRV
Answer
The response has no answer records.
Authority
fastly.net. 30 SOA ns1.fastly.net. hostmaster.fastly.com. 2017052201 3600 600 604800 30
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0