AR-750 Creta Extender mode not working if DNS set

Hi everyone,
I’m experiencing what I think is a weird behavior when I try to set my AR-750 as Extender mode. It happens that I use it to connect to my phone through USB cable so I change DNS settings to Google ones.
Well, it works just fine but when DNS are set and later it happens that I want to use it as extender won’t work: pinging to the main router is OK - pinging for example to is okay but pinging to an hostname does not work - the same result when trying to surf the web.
If I deselect the DNS override setting for all the clients it works just fine in extender mode.
Is that a bug or is it supposed to work this way? If it is a bug I would be curious to know (maybe other people here can test) if other models are experiencing the same issue.

Thank you

EDIT: firmware version is 3.105 modified on Dec, 09 2020 but I was experiencing the same before updating.

EDIT 2: I’ve just tried with an AR300M-Ext and the behavior is the same