Strange wireless adapter problem

[Dairydunkers]

So here's the story, a few months ago I bought an Asus PCIe-N15 on Amazon, it worked decently, about half the performance my router was fishing out but it was fairly cheap so I didn't mind, and all of a sudden, for no apparent reason, the adapter slowed down to a meg and a half, however it is only when connected to my router, when I check the speeds of my router on my phone I'm getting around 50 megs, however when I switch my data on and then connect my pc I am getting better performance agaIn, it is only broken on my pc and only when connected to the router, could someone please tell me what I've broken and how to fix it? Thanks!