vSZ-E 5.1 with old Android cant get the LOGIN-Page on Hotspot (WISPr)

unknown

I do use vSZ-E 5.1 with Hotspot (WISPr).

I do have problems, to let old android devices to get connectet. They doent get the typical "Questenmark-Symbol" an the jump to the Login-Page!

Here I do need to connect the WLAN  by hand, but then I do need to open the browser by myself. And then it says.. similar "Webseite nicht verfügbar" / "website not available"...

The Error-Page shows correctly the URL + Port 9998 of my certificate.

The vSZ-E is only connectable by an internal IP (example 10.30.0.10). Hence I do defined in the "my cluster-settings" the "external IP = " - which is the registered URL of my certificate. On all modern iPads & Androids, this works fine, but NOT on an OLD rooted Tablet?

1)
Has this to do with "wireless Client Isolation"?

2)
Or the fact, that the real IP of the URL is not indeed routed back to the internal vSN-Management-Port?