Can't access internet behind proxy

Hi, we have an app which most of our users are having no issues using with (and most will be behind a proxy of some sort)

Some users at a particular site don't appear to have any internet connectivity through the app, though they have confirmed other apps function OK. It seems to be related to their proxy.

They have been asking us if it's proxy aware. On looking into it, I realised the app build config needed updating, so I changed it from Managed to NSUrlSessions (IOS) and AndroidClientHandler, TLS 1.2 on the Android build. This didn't help either. I don't seem to get any information fed back to appcenter either (this is a UAT build)

I saw somewhere that the way the HttpClient is constructed could also be relevant, so I tried out both with and without a client handler in the constructor and tried posting some data in case it was to do with our client handler but no go, though I expected this to fail too seeing as the appcenter is not receiving any info either.

I saw this post https://medium.com/@anna.domashych/httpclient-and-proxy-76835c784eab and tried the steps in there to see if there are any proxy settings detected, but apparently not..

They've added the server IPs we are trying to hit so that shouldn't be an issue.. So I'm now all out of ideas. so any suggestions on what I could do to understand the issue would be much appreciated! Is there anything I could try logging / showing on screen that might give me some idea as to what's not working? Or some other config/settings I might need to tweak? Or something I can ask them to try?

And just to demonstrate, this sort of simple request doesn't work when on their network but does if they switch to mobile data

var client1 = new HttpClient();
            HttpRequestMessage request1 = new HttpRequestMessage(HttpMethod.Post, Consts.ApiBaseAddress + "/testlog")
            {
                Content = new FormUrlEncodedContent(new[]
                        {
                new KeyValuePair<string, string>("info", "Hello!")
            })};