This particular issue has been discussed in the forum many times before. you can use a variety of equipment for it. The only thing you cannot do is use a switch that is only controlled by SmartThings to turn it back on on demand.
Some people do have a switch which runs on a timer.
But mostly it’s easier to just use a switch which can also be controlled without using SmartThings. For example, a WeMo switch allows for independent control.
Here are some of the previous discussions
Different needs, different solutions, but the same basic idea
There are more discussions than these, but they pretty much all come down to the same thing – – use a separate switch which can be controlled even if smartthings is down. Some are fancier than others, some autodetect a power loss, some run on a regular schedule so you get a reboot every day or every week, some are people asking about rebooting the modem rather than the hub itself which complicates the issue and requires a self-starting switch.
A lot depends on whether you want to be able to reboot on demand, automatically reboot after any power failure, reboot the router if the power is restored but The internet/wifi is down, or all 3.
But they’re all similar, just the specific devices under discussion vary depending on the exact features that you need.
The ResetPlug Itself
The specific brand resetplug has also been brought up in the past at least four or five times. This particular one does get some bad reviews on reliability, which I would find a little concerning.
Personally, with regard to resetplug, I wouldn’t use it because of its methodology. It’s continuously connecting to websites over the Internet and I don’t like either excess traffic or some of the security issues that it raises. I would rather just use a device which monitors the power it’s plugged into.
Yes, it’s possible that the wifi could go out when the power didn’t and then there’s an issue. Personally, again, if I thought that was going to be a common occurrence I would get a device which monitors the local Wi-Fi connection, not actually going out over the Internet, and then an external website of my choice on my schedule. But that would probably cost more.
Check the Amazon reviews:
https://www.amazon.com/dp/B01BU2ALGO
If you do need to track Internet connection, not just power outage, I prefer the following brand to the resetplug because it has more features that I would use. In particular, I get to specify the website it pings, which, like I said, is a security advantage over the resetplug brand. It also has some nice timer options which the resetplug does not have. This brand has much higher Amazon ratings, and I think these features are probably why. It also seems to be somewhat more reliable.
On the other hand, it costs more. So as always, different things work for different people. 
(3G Store has now rebranded themselves as 5G store.)
They sell several different versions of this:
Also here’s a very stripped-down version from a different brand. No notifications, only one socket. But it does have pings and a once a day timer for under $50.