Broadband speeds

Can anyone give me some advice? I was on BT fibre until earlier this year then I upgraded to BT Infinity and was told I could get up to 76Mb download and 19 up. My iPhone shows I get about 66 down and 19 up most of the time, but sometimes it can be really low (around 5) then I reboot the router to restore the speed. That's on WiFi obviously. My laptop, also on WiFi reports around 35 down and 16 up on WiFi, my desktop, linked by TP Link through the mains reports similar values. Should I expect better, or am I getting good results?
I am using Windoes 10 64 bit on laptop and desktop with Firefox as my browser. iPhone has latest IOS and safari as the browser.
Appreciate all comments.

You need to test with an Ethernet cable direct to the router, not through powerline adaptors.

Tried connecting the laptop to the router using ethernet cable, similar results. Used the BT speedcheck which shows speed at the hub is around 62 at that time, although I have recorded higher. Normally use Ookla speed test on all my machines. Must be a bottleneck somewhere, but where?

Is it only at peak times?

No, all the time. At peak times I get dropout as well.

Time to see some line stats from the router, and a description of how it's all set up in your property please.