If the range is 34-36 then a guarantee of 39Mb cannot be correct .
With your current deal , you presumably were given a minimum speed guarantee, that won’t necessarily be the same as any estimate for ‘new’ service, if your connection has generated real world data , that would form the basis of new estimates, rather than some mathematical calculation used to provide the initial estimate, line estimates are dynamic they do change.
Your router stats will give the best indication of your line potential, if it’s underperforming and all possible ‘internal’ issues have been eliminated, like running the router from an extension socket , or running from the main socket but having defective wiring or sockets or equipment also connected, then report it to your provider, if the speed you get is below optimum but still in excess of the minimum, your provider may not consider it a problem.
If you are a BT customer , post your router stats
Sorry type minimum guarantee is 30meg the router stats are below. I'm connected to the master socket . I've attached the email