For some time we have been investigating an issue where Zoiper might be unable to register unless the phone is rebooted after change to Wi-Fi or the Wi-Fi hotspot.
It turned out to be a bug with Android itself. This bug prevents the phone from registering on servers using standard port "5060" and TCP connection.
As workaround you can ask the VoIP service provider (PBX administrator) to allow registration on non-standard port, use UDP or just 4G.
This has been reported to Google in 2017, unfortunately it was not resolved. Currently there is not much we can do about it.
Technical details and detailed description of the issues can be found on the links below: