@nickrout
Thanks for the feedback.
Edit 1
I think the problem is related to SSL certification. So I created a self signed certificate like shown in this post.
Then I tried to connect to ZeroTier with an Android phone and a Windows PC and both of them could access HA with just a certificate warning.
So the problem is with my iPhone. A quick google search unearthed many issues between iPhone, SSL and HA but I am still looking for a solution.
Edit 2
By reinstalling the ZeroTier App on my iPhone I could finally access HA while connected to my WiFi network.
However, I lost access as soon as I switched to Mobile internet.