How to automatically set a default route on RPI when ppp0 (3G) connection is established?

I want to connect a RPI to the internet via an UMTS stick. I could get it working with some manual interaction. As the RPI is place on an external location where i don't have access to all the time, it should completely "come up" automatically when its booted.

What is did:

sudo apt-get install wvdial ppp sg3-utils

set /etc/wvdial.conf to:

[Dialer Defaults]
Init1 = ATZ

; Huawei E182E:
;Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0

; Huawei E173
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2

Init3 = AT+CGDCONT=1,"IP","web.vodafone.de"
;Modem Type = Analog Modem
Modem Type = USB Modem
ISDN = 0
New PPPD = yes
Username = ''
Password = ''
Modem = /dev/ttyUSB0
;Modem = /dev/bus/usb/001/011
Dial Command = ATD
Stupid Mode = 1
Phone = *99#
Baud = 115200
Auto Reconnect = on

set /etc/ppp/peers/wvdial to (no idea if this is required and what this does?):

noauth
local
name wvdial
usepeerdns

created /etc/network/interfaces.d/ppp0 (i have the impression that this does nothing as it does not "up" the interface at boot and the route is not set)

auto ppp0
iface ppp0 inet wvdial
up ip route add default dev ppp0

as the above interface configuration does not activate the interface at boot, i added this to /etc/rc.local-> wvdial&

When i (re-)boot the RPI after all these settings, the interface is properly activated but my traffic is not routed to the UMTS interface. When i execute ip route add default dev ppp0, everything works like it should be and the UTMS interface (ppp0) is used for internet traffic.

How can i automatically add this default route when the device is booted? or maybe better, when the ppp0 interface is connected?