@4barrel 

Several questions...

1.  If you connect your PC directly to the ISP modem/router, what speeds do you achieve?

2.  Are the connections directly wired or are there intermediary devices, such as a switch?

3. What type of cable are you using for the connections (i.e. Cat 5, Cat 6, etc.)?

  @jra11500 Hello, and thanks much for your questions. Answers:

1) Direct connect to ISP modem I achieve the same speeds 550-650 range.
2) I do use an ethernet dongle (uni) that connects via USB C to my mac laptop (2020 M1 processor running latest OS). I swapped with a belkin dongle and it did not make a difference.
3) I'm using cat6. The ISP router is in the same room and I have a 10' cat 6 cable running to the uni/mac. Inside wiring from exterior network interface to ISP router is fiber, installed by ISP.
Lastly, I have run speed test on wifi, and it's basically the same.

  @4barrel 

If you connect your PC directly to the ISP modem and the speeds are the same, it would appear that the problem is with the ISP and not with the AX55.  It also could be with the interface adapters.  The Belkin 2.5 Gb USB-C to Ethernet adapter (from what I have read in the apple forum) is very capable of doing 1 Gb speeds.

If you have access to another PC, I would try it to see how it performs.  That may help you narrow down the problem.

  @jra11500
Thank you! I will try another PC. The ISP says they'll charge for a site visit, so I want to try as best I can to eliminate all possible issues on my side of the fence.
Will report back when I know more. Thanks again.