Zyxel NR5103E giving very poor speeds

[Lags]

Our speeds with this router were fine a few weeks ago but these days it’s really hard just to get one laptop to be able to watch YouTube at a relatively low bit rate. We are not able to receive 5G where we are for some reason, but we can get 4G and the Three app is telling me that the 4G signal is excellent. So my question is - what is going on and why can’t we have more than one device connected to the Internet without suffering terrible speed?

I have tried restarting it many times and moved it to the best position available. 
It’s not acceptable anymore.

[Lags]

We use Apple devices. According to Three, it's a mast contention problem. Will cancel next month if it isn't sorted.

[Lags]

We use Apple devices. Three have admitted that there is contention on the local mast and are supposedly trying to syphon off some of the connections elsewhere. Still getting 2Mbits/sec at times. Will cancel next month if there's no progress.

[MymsMan]

Most of my speed problems are on the local Windows wifi side rather than due to the 5G signal, a wired connection gets consistently good speed (300-600Mbps) while wifi connections drop to less than 40Mbps at times

• Speeds are always slow when awakening from sleep, Disabling/Enabling  the wifi adapter restores full speed, I have created a simple script to make doing this easier each time I wake up the computer.
• I have also disabled TCP IPv6 on the laptop -  it seems to cause variations in download speeds