So confused about fibre - keeps reverting to ASDL speeds

[#1]

So I 've recently switched to Fibre and was delighted to initially see speeds of around 73Mpbs. However, within an hour, I was back to my previous ASDL speeds of between 0.5 and 10Mbps. I managed to speak to an advisor who said it was probably due to using a wifi booster (so it can reach the upstairs bedroom) and that I needed to reconfigure the router so that the the 2.4gHz and 5gHz channels weren't synced. 

That also worked for a bit, we got back to speeds of c. 70Mbps and have realised that we'll just have to give up on the idea of having broadband upstairs.

However, I've now sat down for work this morning and I'm back to speeds of 0.5 to 10Mbps and my wife has had to move rooms as we can no longer can get wifi in her office (spare bedroom).

Any ideas what could be going on? Could this be linked to us both being on VPN?

Thanks in advance.

[#2]

With the VPNs and wireless booster switched off, what’s your connection like on either the 2.4GHz or 5GHz in your wife’s office or upstairs?

If it’s poor and with the VPNs still off, try turning the wireless booster back on and connect it to the 2.4GHz and/or 5GHz in turn. 

Also do you get good wireless coverage in any of the rooms in your house?

[#3]

Hi Thrall,

Thanks for replying so quickly. I just ran a test (wireless) from the room adjacent to the router and we're now seeing c. 70Mbps before I'd disconnected VPN. 

Speeds are still c. 1Mpbs upstairs and devices tend to switch back to 4G instead.

I'm wondering if this is the "settling" they talked about. I just expected that would be a variation of about 10 Mbps, not between 0.5 and 73Mbps!!!

I haven't noticed any difference between 2.4 and 5, except that in the room furthest from the router (wife's office) devices can only seem to find the 5gHz network, which surprised me!

Thanks again.

[#4]

The settling/training period doesn’t apply to wireless. It’s applies to the broadband signal into your router which wouldn’t be relevant here as you’re getting a good speed in some rooms of your house.

Is there any difference with the wireless booster off?