15mbps is on on the very low edge of being able to stream. Does your provider offer low bandwidth channels? If so, use them. Also, try staying away from anything with an HD in it’s description. Try using only SD streams. If you can connect using an ethernet connection; do so!
Btw, kill ALL background apps running and clear cache on ALL apps. A vpn will usually hinder (but not always) the speed of your signal. So if you are using one try turning it off to test. If you live in an country that one is not required. Then I wouldn’t use one. Where I live in the states I use one to deter Concrap from throttling me, but in your case if you get throttled you would just about be at around 5-7mbps, and that spells disaster for streaming.
fyi, since you live out in the sticks. Have you researched StarLink? They had a beta program that claimed speeds of 50-150mbps. There is also Hughes Net, Viasat, and several others. Hang in there 1,000’s more satellites are being launched every year. Imho, I believe this is the wave of the future for internet connects worldwide!