VPN Issues w/ D-Link DSL-502T

I've setup a small office in Andheri(E). We used to have a cable connection via some local guy and his connection became slower and slower daily, so I've setup a MTNL broadband connection.The speed is good. 512kbps tests to 400 up / 400 down w/ dslreports.com.My problem is that the VPN I need to connect to which uses IPSec and a Cisco software won't connect unless I switch from TCP to UDP and w/ UDP, only 1 person can use the VPN at a time. If someone else turns it on, the other person gets knocked off.I've inquried about buying another DSL router/modem, with no such luck. If anyone knows where I can get a later model ADSL router/modem, please let me know.If anyone knows a solution to my problem, let me know.I have a couple of wireless routers from Linksys and Netgear that support PPPoE, but I don't know how to turn off PPPoE in the DSL-502T and turn it on in the other routers.I've also applied for a Tata Broadband connection and am hoping their DSL router/modem will be a different model and will actually work.