Slow (Indirect?) Connection even under DMZ

Ok, so this has been a problem since circa version 11 of TV. In 2016 and back I didn't have this sort of problem but after some updates I never could get a direct fast connection between my two computers..

I have two houses in different cities (Santos and São Paulo in Brazil) both using the same ISP provider and connection with a high quality 50/25mbps Optic fiber link (fiber directly into the modem). I get excellent 25megabits per second upload speed in virtually every different service, be it simple file uploading through browser, sending files through skype, bittorrent sync, ftp, google drive etc. This on both pcs. UPNP works as advertised but I have even set up fixed ip on the DHCP of the routers of my both homes to set up DMZ and nothing helped. 

When I try to upload a file from on computer to another, or access files via VPN the connection works as 25mbits for a few seconds and quickly drops to about 1mbit (around 125kilobytes per second or even lower). 

Of course I tested a direct simples ftp connection between the two pcs and the speed was perfect 25mbit/s as advertised. I can't fathom why Team viewer can get a direct connection to work with my both pcs, it seems as the bandwidht is being rerouted. I don't believe my ISP is traffic shapping VPN connections, it makes no sense for them to do so, Can't I manually configure ports or something for the VPN connection between my two pcs? 

Both are Win 10 running TV 13. 

Thank you.