Smart 5G Hub and wifi range extenders

[peterk]

Background. Currently on BT FTTC broadband.  Currently upload speeds around 0.8 Mbps and BT think this is line degradation and Openreach cannot be made to address this issue.  Guaranteed download speeds are met. Since I don't need a landline , my EE "indoor" 5G signal  is "excellent" and I'm out of the BT contract then moving to EE 5G is justified on cost and performance grounds.  I currently  have whole house wifi coverage with the BT hub and BT wifi disc ( only for 1 critical room).  EE have given mostly unhelpful information ( best being "move the router when you change rooms" ) and the 2 pages I've found for the hub on EE.co.uk have little technical information and hence 3rd party wifi range extender compatibility is unknown ( EE offer a £7 / month range extender but definitive compatibility cannot be proved

Question.  Are there any known solutions to meet the issues above?  

[XRaySpeX]

EE offer of a £7 / month range extender applies to fixed BB, but I don't know that it does for mobile BB.