@OR_or 

Could you please tell me how far from the adapter to the router? And please tell me any obstacles between them?

What's more, could you please try this firmware can help you solve the problem or not.

Download link:https://static.tp-link.com/2019/201911/20191128/T4E.zip

Have a good day!

@OR_or 

the solution is not to use 5g with this adapter if there are any obstacles between router and pc. its super unstable.

for example i cannot even stretch my legs under the table (on 5g wifi), it loses connection immediately.

or you have to tune this antennas perfectly and dont touch it if it works. ive managed to do it only once. now after ive done some cleaning i cannot tune it to be stable again (from same spot) and i just gave up after 2 days of experiments and locked it to 2.4g channel. only ~30 mbps of download speed (100/100 internet) but atleast its stable.

@fnam 

This is because the signal attenuation of 5G will have a greater impact than that of 2.4g, so it may lead to a more stable use of 2.4g when there are obstacles. Of course, as you said, it might be helpful to adjust the direction of the antenna and so on.

Have a nice day!

@Kathy_Z 

I was getting very low speeds, around 800Kbps, but have got it up to about 5Mbps by moving the antennae while downloading a big file. My laptop gets 20Mbps from the same location, which is 10 meters from the router and through 3 walls, so it's still not really good enough, but I thought this might help someone.