I have a similar problem, It seems that switching connections (i.e. a handover between a spotty Wifi and 4G) sometimes may cause Zoiper to fail to register, but when Push is active, the problem becomes exacerbated. The phone indicator claims registration is successful, but it is not, and an attempted call is rejected by the SIP server because of no registration.
Without Push, it seems Zoiper is more able to resolve the issue and re-register successfully, while with Push, it remains unregistered until restart, and with no indication that incoming calls are unsuccessful.
For me, this precludes the use of Push Server. In fact I lost my money paying for the service that I thought would be great, but in fact creates trouble.