You are replying to a post which is two years old, so there are definitely new recommendations since then. 
Since the smartthings motion sensor is zigbee, you will need a zigbee device that can repeat.
At the time of this writing, July 2019, one of the most popular is the IKEA Tradfri plug-in pocket socket. This is just a simple on/off device, no energy monitoring or special features, but it is available in many different countries and if you buy it directly from IKEA, it is only $9.99 in the US or £9.99 in the UK. 
See the FAQ for more options:
FAQ: Which devices work/double as a Zigbee Extender? (2018)
That said, while it’s always a good idea to strengthen the mesh, The “going off-line” intermittently issue is more likely to be a different, cloud-based problem. Usually if an additional repeater is needed the sensor either doesn’t seem to work most of the time or it goes through batteries very quickly. The “smartthings marked it as off-line” is a different issue having to do with smartthings’ own device health” feature and is much harder to both diagnose and fix.
So while it doesn’t hurt to add another repeater, I’m just not sure it will solve your particular problem. 