I agree that the best approach here is to strengthen the mesh by adding repeaters of the appropriate protocol.
Zigbee repeats only for Zigbee and Z wave repeats only for Z wave so you need to get a mains powered device of the same protocol as the sensor placed about halfway in between the hub and where the sensor is. (Battery powered devices do not repeat) popular options are pocket sockets, plug-in motion sensors, in wall relays, and light switches. (Some lightbulbs are will also work, but that gets more complicated just because of the variety of protocols .) pocket sockets are typically the least expensive and the easiest to move around.
After you place the repeater where you want it, you need to get the sensor to update its neighbor tables so it knows who its nearest neighbors are. This requires doing a Z wave repair utility for Z wave or unplugging the hub (including taking out the batteries) for at least 15 minutes for Zigbee devices. That will send out the message to tell the devices to rebuild their own neighbor tables, but it can still take a few hours before each device finishes updating so you may not see improvement until the next day.
Back to the original question, which may not actually be relevant…
The longest range sensors which can integrate with smart things are the Kumostat wireless tags. People often use those if they need to reach an outbuilding more than 200 feet away. But they are not likely to be the right match for your particular situation. I just want to mention them in case people in the future find this thread based on its title.
For sensors inside a single Home, the longest range will usually come from zwave plus devices which have a range about 50% longer then classic zwave or Zigbee. Zwave plus is fully compatible with classic zwave so you can have a classic sensor and a plus repeater or vice a versa and everything will work fine .
Before even trying a repeater, though, you might want to just update all your network tables by running both A Zigbee heal and A zwaverepair. One of those can’t hurt, might help steps that can greatly improve the overall efficiency of your existing network.
For more information, see the range FAQ: