|First off, if you're only using one bandwidth test site (speed is not the correct term.....google the definition of bandwidth and you'll understand why - sorry but it's a pet peeve) then I wouldn't be too concerned. If it were me, I'd try 3 or 4 and compare. The overall average of them should be close to what you're actually getting for bandwidth.|
Keep in mind, results will vary with time of day, traffic on the internet, and number of hops between you and the test site. So maybe try your tests at different intervals. Say, early in the morning, in the afternoon, and late and night. Again, compare results.
Keep in mind you lose a percentage of your rated amount of bandwidth due to overhead. This is normal. 10 to 20% is expected. 50 to 60% is not. So if you're only getting 40 Mbps on a 100 Mbps package, you need to contact your provider because you should be able to consistently get around 80 Mbps.
Instead of carting your PC downstairs, could you get ahold of a cable long enough to go from router to PC? If not, how about getting a friend with a laptop to come over and connect them to the router with a cable. It's best to do this with a computer that's actually plugged into the network with a proper patch cable as wired will always outperform wireless.
The results you got with your phone are erroneous and misleading, ignore them, they mean nothing.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.