EX6410 — AC1900 WiFi Mesh Extender

Have Virgin Broadband which usually works really well and fast. Bought 2 EX6410 - AC1900 extenders to boost signal around house and since then the BB speed has plummeted to virtually none existant. Before I return them both, any ideas whether I can still keep the extenders and improve the signal? (I have no techy knowledge whatsoever so please respond in laymans terms thanks!!)