The Samsung SmartThings hub (usually called V2 in the forums) comes in two different models. One only supports the US zwave frequency. The other only supports the EU Zwave frequency. Neither can be switched to the other. That’s just how most Zwave controllers work.
The hub does include two different controllers, but one is for Z wave, and the other is for Zigbee. This means you can have devices of both these protocols on your account, regardless of whether the Z wave controller is on the US or the EU frequency.
Zigbee is the same in both countries except that US regulations allow the radio frequency signal strength to be higher. So zigbee devices manufactured for sale in the US often use what is called “boosted” or “amplified” Zigbee, which would not be legal for operation in the EU.
There are many battery powered Zigbee sensors available, in fact I think most of the ones that work with SmartThings are. So it may be that what you were actually trying to do is extend the zigbee mesh. I mention that because zigbe devices can only repeat for other zigbee devices, and Z wave devices can only repeat for others zwave devices. So if you are trying to use a Zwave device to repeat for zigbee, your network is probably going to break.
The first generation SmartThings motion sensor which was USB powered could act as a Zigbee repeater. But I don’t think it’s sold anymore. A few retailers might still have it. It is the one with the micro USB power port.
Alternatively, you should look for a zigbee plug-in device that works on your voltage that uses the zigbee home automation profile (usually abbreviated ZHA 1.2). Those have the best chance of working with SmartThings.
If instead it does turn out that you are in fact trying to extend a US frequency Z wave network, if you can find a Z wave lightbulb by linear, Domitech, or Aeon labs that works in your fittings, those will act as zwave repeaters as long as they are on the same frequency that you’re using.
These lightbulbs are often useful to get signals up stairways where there are no places to plug things in.
Note however, very important, zigbee lightbulbs like Hues will only repeat for others zigbee lightbulbs. (There are some zigbee light bulbs that are supposed to repeat for sensors, but forum members report it’s difficult to tell which will and which won’t except that Hues won’t.)
Anyway, the first question is exactly what protocol are the sensors using that you have been losing connection with? Are you sure they are US zwave and not zigbee?
If they are US zwave, A zwave lightbulb might work.
If they do turn out to be zigbee, you’ll need to look for a zigbee repeater, but probably not a lightbulb. The Samsung plug-in outlet made for the UK is 220 V and is a zigbee ZHA 1.2 repeater, but I don’t know if it fits your country’s receptacles.