Night Hawk WiFi Range Extender AC1900

I have a customer who uses Virgin Media broadband and a couple of years ago I supplied him with a Night Hawk Wi-Fi range extender which was installed at the opposite end of his large split level house from the location of the Virgin router, it connected to the Virgin router via a couple of plug top devices utilising the mains voltage wiring in the house and it was configured as an Access Point. He was happy with this arrangement and for the most part it covered the house with a very good to good signal. But recently he has been talking to a friend and as a result expressed an interest in changing the arrangement so that there was a single Wi-Fi broadcast name and so that he would not have to swap between the connection provided by the Virgin Wi-fi and the connection provided by the NightHawk when moving around his house. To satisfy his request, I suggested that we could re-locate the Night Hawk to the first floor of the house and connect it to the Virgin router via a UTP cable. Set it up as an access point, switch off the Virgin media wi-fi signal and then install a further Night Hawk on the ground floor also connected to the Virgin router via UTP. I thought that it would be OK to have the two access points set up with the same SSID but no matter what configuration I try the two access points do not work together. Upstairs the signal is good and quite stable but downstairs the signal works, drops out, works drops out. Sometimes it works for hours at a time sometimes for just a few minutes, sometimes not at all.

Any advice as to how I might resolve this issue would be greatly appreciated.