@Costas-D 

Hi,

There are several models of PLC's available that support TP-Link's own "OneMesh" technology and some of them even support standard "EasyMesh", both of which would provide for fast roaming.

However, Wi-Fi PLC's and Wi-Fi Range Extenders cannot build such a Mesh network on their own. A main router that supports the same Mesh variant as the PLC or the Range Extender is required.

Therefore, please check if your non-TP-Link modem router maybe supports "EasyMesh". If it doesn't, then this can't work.

In case your non-TP-Link modem router supports another Mesh variant that is specific to that manufacturer, then you could check if that manufacturer also makes Wi-Fi PLCs that support the same Mesh variant. 

  @Costas-D 

Seems a new product PGW2440 KIT has been released recently that supports EasyMesh for fast roaming. In the case of your non-TP router supporting EasyMesh, you may choose this KIT. There are two PLCs in the KIT, one supporting WiFi and one doesn't. Connecting the PLC not supporting WiFi to the router such as this.

There will be only one WiFi name (SSID) in your house.

There are other ways availible if you are interested.