Providers only guarantee speeds on wired devices because WiFi has too many variables that are outside of the providers control , it’s not really your provider’s problem if the are supplying the appropriate speed at the router .
If your WiFi ‘signal’ upstairs is poor upstairs, you could try repositioning your router , or look into WiFi hotspots (BT can supply’whole home WiFi’ ) or buy something similar to relay the WiFi signal upstairs and hopefully make WiFi more usable, TBH it’s difficult to see how a reset would improve this even briefly.
By far the most transformative thing to do would be use wired devices so connecting to the hub with Ethernet but obviously this may not be practical for devices like phones or tablets intended to be portable.
