Two things seem strange. First, the value you set for External Address in Asterisk SIP Settings → General SIP Settings did not propagate to pjsip.transports.conf. Also, a domain name in external_media_address did not get looked up. If those worked properly and you have dnsmgr enabled, restarting Asterisk should not be necessary. See
A quick test (not thorough) showed this working on my system (the name got looked up and the corresponding address made it to Contact and c=).