Convert RBR50 to RBS50 : stuck

Hi, 

I know this will not be officialy supported. But I have already converted a RBR40 to a RBS40 succesfully. 

Now , i was in the process of converting an RBR50 to RBS50, and already changed the model ID via Telnet (ok)

The next step is then: reboot and login to the (former) router again, and manualy upload the fimware via the Web GUI

Except: i can not login to the standard SSID (ORbi xx) anymore?

My other RBR50 (router with some satelites) can find it as a satelite (because I renamed it with telnet commands). But of course; the semi converted RBR still has the router firmware on it. 

The Netgear login web page keeps saying: configure synchronisation....but I fear it will not be able because of the wrong firmare.

I have tried FTPT client firmware upgrade: but it will not flash (i connected the RBR-S with ethernet and tried couple of times with the FTPT firmware method: but no succes)

Does anyone has THE solution to get out of this "loop"?