I have 4 PX50 units distributed between different floors of the house. Main unit is very stable, but satellite units are going offline sporadically and frequently. All units are using PLC and Wi-Fi to

Hi All, I bought a set of 3 units of PX50 and love it! The only issue is that I have to cover 4 floor house and need one more unit. Is there an option to buy only one unit? Thanks inadvance, Kind rega