I've been trying to work through this all night, to no avail. I'm about ready to move back to Jessie.
Essentially, I'm trying to setup an auto-ppp interface (like I've done many times in Jessie) and usually, I'll just set this inside my /etc/network/interfaces:
auto gprs iface gprs inet ppp provider gprs
However, I'm reading that in Stretch,
interfaces is now obsolete. This poses a problem as now I cannot setup my interface and have it start automatically within dhcpcd.conf.
I feel as if I'm missing something - but I'm not sure what.
The end of my dhcpcd.conf:
interface eth0 static ip_address=192.168.0.1/24 static routers=192.168.0.1 static domain_name_servers=188.8.131.52 interface wlan0 static ip_address=192.168.1.155/24 static routers=192.168.1.1 static domain_name_servers=184.108.40.206