Eight metres is quite a long way. Mine are 10-15 feet apart. They don’t need to be in range of the coordinator - messages get passed from router to router and the important thing is to make sure there are lots of alternative routes for them to take.
Another thing to bear in mind is that not all routers are created equal. Bulbs are likely to be less effective than plugs, for example. You might try adding two or three “repeaters” (ie routers that do nothing else) - I have at least one in every room.
Has anyone given you this link?