The first two things I thought of are 1) how far away is the mini from your router and 2) what is nearby that might interfere with it? Illaaas and I are on the same page here.
Quick story - I had to move my computer from its original location in the basement after I set up my WiFi wiring closet in the upstairs linen closet. It was a great location for the router, central to the entire house. Unfortunately the main heating duct ran directly underneath the closet and my computer was directly above the duct putting it in a WiFi ‘shadow’.
The original WiFi standard broadcast on 2.4GHz which is shared by lots of other wireless devices including baby monitors and wireless phones. If your router supports both 2.4GHz and 5GHz it is a good idea to configure it to broadcast on both. Older devices can only receive on 2.4GHz so you many not be able to use just 5GHz. In my household this switch made for a nice change on our newer devices.