So I'm just putting this here in case it helps anyone searching for a resolution to this issue. I had this issue with both computers being on the same network with decent connection (200Mbps).
We always had the issue of the game telling us the non-host player had "high ping" and the game was almost unplayable. I tried everything to resolve it and nothing seemed to fix the issue.
I looked into how Grounded did it's networking and found it was peer-to-peer. After searching that specifically, I then found this registry file that a post on a thread, was promising to fix latency issues for an MMO and at this point I was trying anything.
It seems it worked for me and now we always have perfect connection.
I can't find the original post I found it on but here is the file, re-shared. I hope this helps someone as this game is great fun, but not when you can't block, or deploy your dandelion because of high ping
https://www.dropbox.com/s/sr2zamgg4h91z5h/NetworkThrottlingIndex.reg?dl=0