Zigbee is a mesh network, you don’t use boosters, you add repeaters. I’ll post a link at the bottom of this note with more information about how this network protocol works if you’re interested.
That said, the problem you’re describing with the sensors being marked as offline is unfortunately a common problem right now and has nothing to do with the strength of the signal in your home. Instead it has to do with how the cloud platform is monitoring the device. Your best bet is to get in touch with support and see if they can help you with the issue.
And here’s the information about zigbee networks. Start by reading post 11, and then go back and read the whole thread. But again, I’d be very surprised if the information in that thread is relevant to the specific problem that you are seeing right now, given what you’ve already described.