I have recently start using/testing Zoiper (a softphone) on my Android to connect to a TCP SIP server behind a netgate firewall.
Connected with my android in the same network as my server, it works well.
Using my VPN with all traffic being tunneling, it works perfectly under 4G and every home router WIFI (tested with 2 huawei routers with different models and 2 telecom operators).
But using another VPN with just half (the target network being routed, where the server is placed) of traffic being tunneling (created just for the purpose of access this TCP SIP server), it only works under 4G. Behind those two routers mentioned before, I just got timeouts from zoiper app. But at least the webservice hosted in that machine, is reachable from this VPN in all cases.
I have this zoiper program installed in several Windows 10's working well, but those also use that full tunneling VPN.
I really dont know how to solve this, I have already verify all options on Zoiper but cannot find anything related, and VPN seems to be working as expected. Not sure if could be some issue under Android level. I didnt had enough time to test this half VPN in a Windows 10 machine.
P.S. if you need more info fill free to ask.
Thanks in advance!