It depends on what the ISP put in the contract as to what your upload speed should be. 3mbits seems very slow but you need to see what number you are suppose to get.
You always need to do your testing on a ethernet connected machine. If you have a separate modem and router you are best plugging into the modem (you likely have to power cycle the modem). You are doing all this to prevent the ISP from blaming your equipment for the problem. It is extremely common for wifi to have all kinds of issues.
It really doesn't matter if you are the only one using 5g in your house. You may have 20 neighbors also using 5g wifi on their router interfering. In general the poor upload speed means the router is having issues hearing your PC. This tends to be common since routers put out maximum legal power but many end device have tiny antenna and low power radio transmitters. You should get better speed by placing your pc next to the router. You can do it when you are doing your first test with the ethernet connection. If you still have issues next to the router I would first see if there is a driver for your nic card but after that you start to suspect a defective nic card.