@Kevin_Gozk 

Have you've been able to solve the problem by now?

I've managed to borrow a RE305 V4 that was on firmware 1.0.11. 

First I tested it with firmware 1.0.11 and then upgraded it to the firmware 1.0.23 downloaded from here.
 

With both firmware versions everything worked as it should. 

The extender never automatically copied the router's SSID names to the extended SSIDs.

By default the extender adds "_EXT" to the router's SSID names and I can then manually change them to something else without problem. 

I tested it via the browser webpage as well as the Tether app and connected the RE305 to a TP-Link router as well as a router from Huawei (but different model than yours).

  @woozle 

If I was able to solve the problem, the solution was to change the firmware to a previous one, with that the repeater went back to working normally 

  @Kevin_Gozk 

Ok, that's good to hear.

Although it is weird that the firmware 1.0.23 you've had installed behaved different than the firmware 1.0.23 that I downloaded and installed.