Slow upload speeds when hosting files

Hello! Not so long ago I was ignited with the idea of personal home server to collaborate with my colleagues on projects.

Eventually bought 3 separate internet connections with dedicated IPs from different providers. We ran multiple speed tests on different Windows 10 machines using multiple protocols like FTP. But unfortunately all of them reach only 3MB/s max! Which is unaccaptable for large file syncs.

The interesting part is when you try to upload a file to cloud server, the upload speed is 12MB/s. But when you try to download a file directly from the computer - it's around 2MB/s depending on the internet. The computers are now connected directly to the internet, without any routers for now.

What am I doing wrong here? Is there any differences between hosting a file and uploading it to FTP server? Is Windows limiting upload speed?

Thanks for help!