If you want something with professional monitoring, you can use Noonlight , but you are still totally dependent on the Internet, there’s no cellular option.
[OBSOLETE] Professional smart home monitoring for security and smoke alarms with Noonlight
If you just want a nice keypad to use with smartthings with the official smartthings home monitor features, no professional monitoring, rboy has a DTH for the ring security GEN 2 keypad, which is a nice device. But you have to pay for the one time rboy subscription and you still aren’t getting cellular communications. It’s just smartthings with a keypad.
[RELEASE] Ring Alarm Keypad (2nd Gen) Device Handler
But personally, I just wouldn’t use smartthings for security in any configuration. It’s lacking basic features. Like there’s literally no way to arm/disarm locally. There are lots of other good candidates out there for a low cost DIY purpose built security system with optional monitoring, including ring and abode. There’s limited integration between those and smartthings, but again, ST just really isn’t meant to be a security system now that the ADT dual logo system is going away.