I found a lot of complains about the Sagemcom Fast 5688W Gateway with VPN. I have the same problem. My firmware version is SGJi10202-1.2.74.

VPN does not work from my Windows PC, but it works with my iPhone and iPad!!!

I found out that it could have something to do with the MTU packet size. This MTU value can be changed under Windows. Until now I didn’t have time to dig into this. Does anyone know how to do it and tested this?

The firmware of the Sagemcom Fast 5688W Gateway is absolutely poor. I think everyone should contact T-Mobile and complain about this firmware. T-Mobile needs more pressure to enhance this black box.