I'm not sure I understood the whole thread correctly but, if you haven't tried with cable, try it. Wireless can be very problematic sometimes and can be affected by many things.
My first step would be to discard a problem caused by WiFi. Sometimes being too close to the modem makes more harm than good. If your connection is steady with Ethernet but fails with WiFi then we've found the problem!
If not, then we can proceed to make tests over the ethernet to figure out if there's latency, bufferbloat, dropped packages or any other anomaly that can degrade your internet.