I'm Greg, a volunteer installation specialist and 8 year Windows MVP, here to help you.
Have you confirmed the speeds with a Speed test like speedtest.net?
Are the speeds the same with both wireless and wired connection directly to the modem?
What speeds are other devices on the network getting?
If the problem is the network then I'd contact your provider to ask for help getting what you've paid for. They must make this right.
If the problem is the PC then run the Internet and Network adapter troubleshooters at Settings>Update & Security>Troubleshoot.
Go over these other steps to troubleshoot the network connection: https://support.microsoft.com/en-us/help/10741/...
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.