@azzido Make sure the device is in wireless router mode. On Advanced tab you should have a Security => Access Control option which will allow you to configure whitelist of MAC addresses.

@azzoug RE600X or RE605X are a good candidates too since they are rated AX1800 just as your VX1800v.

@EliWeizman Did you reset the VR600 before you start with the configuration ? Disable VR600 DHCP server and its IPTV. There's nothing more you can do except to update VR600 firmware to the latest one...

@jonl1969 The cheapest solution would be to run an ethernet cable from your AC750 to the inside of your garden room and use another RE (in AP mode) or even a cheap wireless router (in AP mode). This...

@EliWeizman What method did you use to configure VR600 in AP mode ? Here's the oficial method to do that - if you used another configuration, it's recommended to reset the VR600, configure from...

@Asad_Afeef Follow this thread's OP actions and check if you can mitigate this issue (start with FW update from here). If these don't work for you, follow the same thread's last post suggestion.