Tired of using mobile data and hotspot, need advice on why my router doesn’t give good speed.

We just bought an internet sim data, and it works awesome on mobile giving speeds over the 100mbps but when I put the SIM in router, the speeds drop to 4-7mbps and the internet gets too slow.

I mainly use internet on my pc, and I’m tired of using hotspot on my iphone which makes the phone really hot.