set ppp0 as default gateway permanently

Question

I have a Zero W with a 4G module, to use the sim to access internet I did:

pi@raspberrypi:~ $ ip route
default via 10.111.100.1 dev wlan0 proto dhcp src 10.111.100.143 metric 302 
10.64.64.64 dev ppp0 proto kernel scope link src 10.82.126.194 
10.111.100.0/24 dev wlan0 proto dhcp scope link src 10.111.100.143 metric 302 
169.254.0.0/16 dev wwan0 scope link src 169.254.170.232 metric 203 

then:

sudo ip route del default via 10.111.100.1 dev wlan0
sudo ip route add default via 10.64.64.64 dev ppp0

and it works, but after reboot I get the original situation...

I tried to set auto ppp0 in /etc/network/interfaces

I tried to set metric in /etc/dhcpcd.conf

but none worked.how can I do that?

also I have a PPTP VPN, how can start it automatically on system boot?

I'm on Raspbian Buster

Answer 1

I don't know what's your setup and what you have tried. But with a default networking setup a ppp connection will give the interface ppp0 on the RasPi an ip address and a default route to the gateway of the ppp device by DHCP. So you should check why your ppp device isn't giving a default route.

In /etc/dhcpcd.conf you can also define static default routes with option static routers=. So ensure that all these options are commented to not overwrite the default route from the ppp device.

Look that /etc/network/interfaces does not contain any stanzas to configure ifupdown. This may have side effects to dhcpcd.