From your post you are obviously aware that the distance between the router and the computer affects performance, as do the materials used in construction and the position and orientation of the aerials used.
Having said that, when you say you have 30Mbps broadband do you mean that that is what is coming to your router through the telephone line, or is it the famous 'up to 30Mbps' as quoted by your ISP and which you are paying for ?
The best thing to do before you get into an argument with your ISP is to run a couple of checks on broadband performance. One very simple one is to run the speedtest.net test. All you have to do for a spot check is to enter 'speedtest.net' into your address bar and click 'go'. Once the page has loaded you can run the test. You don't have to register to get a spot check, but if you do register you can get a log of performance over time. Speedtest is not totally accurate, but, especially over a series of readings, gives quite a good picture of the service you are getting.
The other test is to look at the performance logs in your router. You can access these by typing the IP address for the router into the address bar of your browser then clicking 'go'. You will need the user name and password for the router once you get to its log-in page. A very common IP address for the router is 192.168.0.1 but there are others.
If either, or both, of these tests show that you are getting service significantly worse than your quoted 30Mbps, then you have grounds for taking it up with your ISP. We had a problem with ours - service fell to virtually nothing - and it was eventually tracked down to a cracked aluminium wire in the phone company's junction box about 400 yards away.
If however these tests show that you are getting service reasonably close to the quoted level, say 25Mbps and up, then it is time to start playing around with your equipment. I would look at aerial position first. I use a USB dongle mounted on the front right corner of my tower and if it slips down by as little as 1/2 inch I loose about 25% of my signal.