add satellite , reposition, or upgrade desktop

I'm embarassed that I did not read the original post carefully.  A six year old computer does qualify as "not new", but is not ancient.  801.11n should be capable of more than 39mb.  If the iPhone achieves 400mb close to the computer, that would indicate that the 5G signal is good and that a 5G USB WiFi adapter might be a quick way to solve the problem inexpensively.  Amazon has them for under $25.  (Just personally, I would go with a "Brand Name" like TP-Link.)

802.11 can be compromised when there is a device somewhere on the network that is only capable of 802.11b.  That forces the entire network to drop to 802.11b speed.  A thermostat would not suffer.  A computer certainly would.