Download speed limited to 27mbps. Need help!

This is my result between my router and my PC. How do I test between router and isp?

Well that tells you that your PC gets 95 Mbit (you can try the reverse direction with -R) and you might want to increase the duration with -t 60

If both values remain in the 100 Mbit area it tells you your wifi performance is fine.
For testing your ISP speed you can either use what you have used above or installing speedtest-cli on the router.

No no, I am connected to the router using an ethernet cable. My router's wifi radio is used for connection with the ISP (client mode). Should I change my router to AP mode and run the test again? But if I do that, there will be differences, my PC doesn't even have a 5ghz receiver. :confused:

Sorry that part I must have missed.
Then the testing approach wouldn't work.
Ignore my ideas.

Thanks anyways mate. By the way, if I want to reverse the iperf, is the command iperf3 -c -r

No the -r has to be before the -c as the -c refers to client and need to be followed by the server ip
Also needs to be capital -R

Don't run iperf on the router, it's not powerful enough to handle iperf, results will be inaccurate.
Try configuring your router to AP, connect two certain devices over wifi and run iperf on those devices to see if the problem is caused by the wifi part of the router.

I mostly agree, but I would propose to run the test with one WiFi and one wired device connected to the router in AP mode.

So, I did what I think will simulate the connection with isp the most. I created a wifi hotspot on my phone, connect to it from the router. This is what I get when they're next to each other. The speed is much better but still somewhat unstable.

But when I move the phone like 20 feet away. The speed gets really bad.

Is it safe to conclude that it is caused by distance?

For anyone who come across this thread and wants to know the solution too. Although I still do not know what the problem was and how to solve it. I did a firmware upgrade from 19.07.4 to 19.07.7 and the issue seems to be now gone. I still haven't had any speed throttling for 2 days yet.