Woo...really strange. You mean all the other ports can work fine only the port 4 have this issue? How many devices do the Dlink connect?

Yes, I have tried this before. This can't be supported by the R600VPN. The only way is set a PPTP Client to LAN VPN.

Hey，dude. I think you let the R600VPN work so hard. As I know from the TP-LinK guys, they suggest the R600VPN to carry 5-30 clients. 80 user is too much.

Hey dude, you mean your PPTP VPN server will stop working sometimes? Have you ever try to reopen the function again. Turn off the PPTP server and turn it on again. What's more, you can try to test if...

Hey dude, Have you ever configure the Bandwidth Contrl?

What's your topology look like? I think maybe some setting was wrong. You can put some screenshots here, maybe I can help you to have a check.