You mean the router? “Bridge” usually refers to a manufacturer’s proprietary box, used instead of a coordinator - the Philips Hue bridge, for example.
In Zigbee terms there are only three kinds of device:
- Coordinator - eg your Sonoff dongle. Only one per network.
- Router - eg light or socket. Mains powered device that also passes messages from sensors to the coordinator. There are a few main-powered devices that don’t act as routers, but most do.
- End device - eg temperature sensor. Battery-powered, does not pass on messages.
A “repeater” is a router that doesn’t do anything else. They can be quite small (and cheap) and they often plug into a USB socket. Manufacturers call them “repeaters” in their adverts, but it just causes confusion.
You might try adding a few “repeaters” if you don’t need any lights or sockets. One advantage they have is that they’re easy to move around, so you can experiment. Check on the forum before you buy - some are better than others.
It isn’t really a question of distance between router and coordinator - that will depend entirely on the construction and layout of your home; also on environmental factors like interference from wi-fi. A healthy mesh will provide lots of different routes for messages to take and Zigbee will automagically select the best one.
I would get three or four and try them out, all in the same room as the coordinator to start with, then if that works, moving them a little further away until you get a good compromise between reliability and range.
You should always pair end devices in the location where they are going to be used.
Good luck! 