connecting to a pptp VPN

A VPN to some servers was configured on Windows 10 clients as 
Type of sign-in info: User name and password
and under "Advanced", 
VPN type: Automatic.

Other windows clients were able to log on to the VPN with no problems, but when I tried with Linux Mint and the default pptp VPN settings, the connection was failing. My first thought was that some setting was missing, but it turned out that the reason for the VPN to fail was that my internet connection was via a mobile hotspot - just connecting via a broadband connection did the trick, I did not have to do any port-forwarding or any settings on the router. And then I found that some mobile hotspots work too - 
LG Q6 with Airtel 4G on primary SIM slot - no
LG Q6 with Airtel 3G on primary SIM slot - sometimes works. 
Redmi 4 with Jio SIM - works
Realme U1 with Jio SIM in secondary SIM slot - no
JioFi router (2018 model) - works
BSNL ADSL broadband with wifi router - works

With the default routing, all connections are routed through the VPN, and normal internet browsing doesn't work. With the help of this post, changed the routing so that only connections to those machines which are on the VPN are routed through the VPN.

/var/log/syslog gave the internal gateway info, which is on the private subnet, different from the external gateway which is on a public subnet.