I can't imagine why but there have indeed been recent reports of new FTTP installations taking a day or so to reach full speed. But you must test over ethernet.
You devices might not be the limiting factor but your local environment might be. You also need to ask yourself what you are doing on a mobile device over WiFi that "needs" 500Mb.
Where did I say I needed 500 speeds for my mobile devices? I said that the speeds on them were disappointing and I have been vindicated because BT have identified a fault and are sending an engineer.
The question I need to ask myself is what speeds to I need to effectively carry out mine and my wife’s business effectively and the answer was 500 hence why I chose it
If you are running a business, then that is not permitted on a BT Residential account. You need to change over to a BT Business account. You will get a better SLA, and can claim for lost business if a service affecting fault causes loss.
You can swap to BT Business, and get any cancellation charges refunded, once your BT Business account has been active for a period.
Its worth reading that whole thread, as it relates to running a business.
There are lots of extra business related facilities on BT Business, which are there to make things run smoothly.
Unlike ADSL/VDSL ( FTTC ) there is no training period where the line is ‘optimised’ , FTTP is not ‘rate adaptive’ , so no variation in line speed , you get what you pay for , however that doesn’t mean there is no possibility of the wrong profile being selected ( so human error ) but that’s unlikely as the FTTP product sold as 100Mb is actually 150Mb , so if you were given that profile your speed is still too low , or it could be , although very rare an equipment fault , your ONT faulty , affecting just you , or an OLT fault affecting all others on the same ‘splitter’ as you .
As pointed out , using WiFi as a speed measure is unsatisfactory, and isn’t normally accepted as ‘proof’ of an issue , measuring with wired devices and/or using the appropriate supplier provided tool is the appropriate method, anyway as your ISP has confirmed an issue hopefully it’s quickly fixed.