very slow 2.4ghz wifi

[#1]

over the last couple of days I've experienced the download speed of my 2.4ghz wifi drop to speeds between 2 Mbps & 7 Mbps.

my ethernet connection is averaging 66 Mbps & my 5ghz is showing around 59 Mbps.

I believe I'm running the latest router firmware on my plusnet hub one.

I've not modified or changed by set up in anyway, from when my system was running fine.

My wifi is split so I can connect to 2.4 & 5 ghz seperately.

I've tried resetting the router, changing wifi channels, removing connected wifi devices, but nothing.

My son complained is PC was very laggy in his room, so I initially thought it was a signal strength issue & installed a wifi  extender. However having spent today investigating, its a speed issue with the 2.4ghz wifi, not the signal strength.

Having a number of devices which only connect via 2.4ghz, I need to fix this issue.

Whats my problem & how do I fix it?

many thanks....

[#2]

D/load a wifi analyser programme and see which channels are most used in your locale;
then select the least used one.
Ensure router's not near a source of other wireless interference eg cordless phone/ NEST~Alexa type gadgets.

[#3]

I've tried these Gel, but without success.

I've tried a number of different wifi channels having moved them to a less populated channels.

My router is clear from other devices. Its in the same place its been for years without there having been any issues (until now)