Just some thoughts (worth what you paid for them)
New logic board and same problem?
It's probably not the Mini (hardware).
It MIGHT be a software setting.
Is the ethernet/USB adapter working well (again)?
What I'd try:
Connect via the USB/ethernet. We know that works.
Then go into the "Network" settings, pick ethernet and click "details"
Then, I'd start "clicking through" each of the items "on the left", and take a screenshot of each pane of settings.
(I wouldn't be able to remember them all later).
Then...
I'd connect using the Mini's internal ethernet port and REPEAT this process.
Once I had the "two sets of settings", I'd compare them side-by-side.
What's the same, and what's different?
I'd pay most attention to the "hardware" settings.
Again, just some thoughts.
May be of help, or may not.