The problem is much worse if individual inverters are used at each panel. Preferred method is to use one main inverter, then noise mitigation only need be applied to the one device.
Please review this story for some ideas. You have to stop the spikes at the solar cells.
If you are certain the noise is being picked up on the Cat cable, you might want to try screened (shielded) Cat 7. This one https://www.amazon.co.uk/gp/product/B08R73J4YG/ref=ppx_yo_dt_b_asin_title_o06_s01?ie=UTF8&psc=1 solved my problem. Given the inexpensive nature of these products, they could be worth a try without losing too much if unfortunately they don't help in your case.
Have you tried running your Flex on a battery? Does that hear it?
Have you tried it with no Ethernet connected? or at least with only short cable between your radio and computer or Maestro?
Is your computer a laptop? Make sure to try it not plugged in,
Basic steps are to isolate everything and then find the source or sources that the problem comes from. Then you can figure out how to fix it.
With solar, does it change between daytime and nighttime? Do you have batteries and an inverter?
