@Hornnumb2,

Is your Router in AP mode?

If your modem is providing a wireless network, and your Archer is in router mode, you will see issues and speed decreases. You will either need to place your modem in IP PassThrough/Bridge Mode - or change the operation mode of your Archer to AP Mode. Difference Between AP and Router Mode
 

Placing your modem in Bridge Mode will remove its wireless network, while using the Archer in AP mode will remove many of its more advanced features and protections, so you will have to weigh the costs and benefits for your specific situation.