One continuous file or multiple smaller files?

One large file should transfer a lot faster.

Point to point WiFi is low cost and works great as long as you have line of sight.

If you want to keep it in house, Mikrotik does a $100 LTE now.

Then in that lte router, you can set up wireguard.

You’ll need to buy a VPS for a few $ a month. Set up wireguard on that, then you can do anything you want.

I can’t find a manual but 4* 1gbps ports just means the sync speeds to your pc.

That absolutely does not mean you can do 8gbps full duplex on all ports at the same time. But simple transfer between two PC’s should easily hit 1gbps.

But 100mbps seems oddly specific. Like a computer was negotiated to 100mbps for whatever reason. Also what was the consistency of the files? Loads of small ones or one big file? Was it the same file set you tested on the tp link?