Gl-MT300N-v2 can't interconnect clients

I am trying to use my little Mango router to cast from my Android 13 tablet to Google Chromecast.
The Mango is in Router mode.
The tablet variously can't see the Chromecast or can see it but gets stuck at "Connecting".
I have tried switching off Randomise MAC addresses.
Any suggestions gratefully received!

Both device connected to Mango?

Yes. Both connected to the Mango wirelessly. Are the clients isolated by default?