I have a weird issue that I can constantly replicate. If I'm on wifi (ANY wifi, not just my home network), I can get to my Sonarr just fine. But when I switch to mobile (4g) I get "Error: null" when trying to show the TV Shows.

I know my setup works outside of my network as I can open a browser from any desktop and get to it.

I am using SSL with a self signed cert and a DNS name instead of an IP. All of the other services (CouchPotato, Sabnzbd, Headphones) all work perfectly fine on mobile.
I finally got around to testing this on another device (HTC One M9) and it has the same issue. Only when on 4g. When on ANY wifi, Sonarr works perfectly fine.
And the URL notation is exactly the same ?

Can you also confirm the port for Sonarr is forwarded by your router. And check Sonarr logs while attempting to connect too.