How to make PPP connect automatically at startup or login

My main (WiFi) internet access is a bit unreliable and I cannot do something about that without spending too much money on it.

So I had the idea to install a backup connection by using a so-called "surf stick", i.e. a USB stick with a wireless 3G phone / data SIM card installed.

I've set it up as a PPP dial-up network interface, like this:

If I hit "Connect", I get connected. When I then disable the primary WiFi connection, I can still remain online thru this backup connection. So far, so good.

Now, my challenge is to have this backup connection to be always connected, even after a reboot of the Mac, e.g. after a temporary power failure.

The problem shows here:

The option "Connect automatically when needed" is disabled! I can't figure out why. Even if I make it the primary interface by moving it to the top of the list, it still remains disabled.

So I ask you all: Is there another way to force a connection to this "modem" interface somehow programmatically? Can I connect via Applescript perhaps? I cannot find any hints on this topic.

(Note: This is a Huawei stick, but using their software Mobile Partner is not an option for me either: Apart from its terrible user interface it also keeps removing and re-adding its network interface, making it impossible for me to arrange it in the hierarchy where I need it as I have a 3rd local network interface which always need to come last - but the Mobile Partner puts its connection always below it.)

I'm running OS X 10.10.5, BTW.