There is your problem with the hotspot. Your Wireless Dongle might say 600mbs on the package, but I would say it probably doesn't even reach half those speeds.
Not sure what to advise you to do
It looks like no matter what way you go you are going to have to spend money. The cheapest way is to get either a good Wifi Dongle or PCIe card. The problem with this is they don't all work with windows 10 hotspots. I guess if you buy from somewhere with a good returns policy, you could try a better one and see does it work. The best option is a dedicated router. But, you have problems if you go this way too. Ideally you really need to be hardwired back to the main router. Since you can't, you might consider buying two powerline adapters.
You could buy a router and try what you said above. But, YMMV if you set it up that way.
