There actually are already seven or eight threads on this issue, as it’s one that a lot of people bump up against.
It’s really simple. You just have to have the right equipment so that you have a means of rebooting the device that doesn’t rely on the Internet.
That probably won’t be SmartThings, though. But as others have said it may be a device which does work with SmartThings as well.
There are even smartplugs made specifically for this purpose which regularly ping the website of your choice and if they can’t reach it then they reboot themselves automatically. Or for some models, on a schedule of your choosing.
If you check the quick browse lists in the community – created wiki and look under project report towards the bottom of that page for the “router” list you’ll find several of those threads. 
Or just go directly to the following thread, which covers most of the options: