@SteMMo 

- is it possible to set an 'internal' wifi password different than the one of the router?
-->RE will has the same wifi password as the router, and this can not be different.

- Is it possible to set a different subnet on the internal wifi net? Now the DHCP is disabled and it is not possible to enable it (why??).

-->No. As you know, even with extender in your network, all the device in your net work should be in the same subnet, extender is acting as a role to forward data between Router and devices without NAT function. Then devices connected to extender can access internet and each other.