I tried connecting to the ONT last week, but the MacBook appeared to not sense any connection. I’ll try again. I’m testing with wifi off… just the Ethernet cable.
I still have the old CenturyLink router and got the idea to reconnect it to see if the slowness is resolved. (its problem was losing net completely, not slowness) So, to get a baseline for the TP-Link router, I ran about 40 online speed tests and only two were noticeably slow (6.3 and 4.2mbps), while the rest were 35-38. Go figure! Hard to troubleshoot when the issue doesn’t reliably show up. But it is often enough for my wife to come complaining to me. 
You said Ziply, previously Frontier, previously Verizon FIOS… We went from Verizon FIOS to Frontier when we lived in the Portland area a few years ago. Haven’t heard of Ziply.
Thanks,
Russell