Phone Client / Universal app can't connect: "Server connection timeout occurred"

Hi everyone,

I have set up a NAV 2016 instance, with Windows authentication, along with the web client. I have installed the selfsigned certificate and can access it via browsers - I get an error on the certificate, but am able to install it and move on.

When I try to open it with the universal app - I have tried Windows Phone, Android Tablet and Android Phone - I always get the error "Server connection timeout occurred" .

I have created a series of different certificates and installed them in the phones all the times: using computer names, internal IPs, external IP,...

Really don't know what the issue may be. I am trying to access via an external IP, but I do get the same error when trying to connect via the domain.

Do you know what this may be?

Thanks