M1 Mini network - HELP!

Some WiFi routers that do "automatic" switching between 2.4 GHz and 5 GHz bands can sometimes be an issue. By the numbers you are quoting, it sounds like the Mini is getting connected to the router via the 2.4 GHz band. In network preferences on the Mini, see what it shows up as WiFi connections. If it is only showing a single network for your router, then this could be the case.

The router may see a low signal at 5 GHz from the Mini and may have decided to set its connection in the 2.4 GHz band. Since 5GHz is "line of sight", anything in between the Mini and the router may be causing an issue (wall(s), power wires in the wall(s), water pipes, even RF interference radiating through power lines, etc.) Having spent a portion of my life as an RF product development engineer, at those frequencies it doesn't take much to screw up communications.

I can only say about powerlines. I have 4 x 600 Mbit in my home. They should rush at least at 300 Mbit/30MBytes/sec. Not sure I can reach 3-5 MBytes/sec. Too many variables are involved in powerlines' performance such as where they are plugged (alone or into a multiple socket?); are there other electrical objects connected nearby the powerline? Are there electric engines such the one in fridges, cloth washers in the same powerplant? And very important: hiw old (or new) and hiw well designed is the powerplant?

In any case I'm in a 46 years old house with all the above described problems but powerlines, IMHExperience are just for "last stage" need. In fact I just use them to transfer the Internet signal around and video streaming (5 Mbit/sec for HD are enough) and nothing else. Of course, the mire powerful 1200 Mbit/sec models in a good powerplant would really perform better.