@jaazz 

TL-WPA4220 has guest network and maybe you could try to let the guests connect to the guest network of 4220, not the main WiFi of 4220.

  @Heidi_Alp 

Thank you for the reply. I already did this, but when the extender (the 4220) connects to the main router, it gets access to the Windows 10 home network that is also connected to the main router.

The functionality I am looking for is for the extender to connect to the main router's guest network. However, this can probably only be done wirelessly is what I am thinking.

My solution is to put a password on my Windows 10 network. Slightly inconvenient, but manageable. This way, when the extender accesses the network, it will have access to media servers (mainly Emby), which isn't entirely a bad thing, but not the other data shared on the network (mainly documents).

  @jaazz That would actually be a feature on the main router. Some can dedicate one of their LAN jacks to the guest network, and then if you connect your PLC pair to that, the far end PLC WiFi will also forward to the main router's guest network only.

  @PeterM. Thank you for the reply. I think the conclusion is that the extender won't be able to do it, so what you are saying makes sense. Will look into this. My main router is very much a budget router, so this facility may not be available.

  @jaazz You're welcome.

It's not about whether the PLC WiFi adapter can or cannot do this. The far end simply is the wrong place to do this

Even if you could implement a guest network there, all the traffic would still have to pass through your private network to get there.

Full separation can only happen if you originate your guest network straight at the internet connection.