@Buddha9008 Hi, I doubt this will be much help but I never resolved the problem with my bulb that wouldn't connect.
I took it to a friend's however and it connected fine to their router/network and worked ok for them - they have the exact same ISP and router model as me so that would prove that my setup should have worked fine. Not the best solution but I think it gave me an indication as to what the problem was.
I think the issue with the bulb I had was that it failed to complete a firmware update properly while connected to my router. After that whenever I tried to connect the bulb the router must have be blocking it from connecting, either because it already thought they was a device using the same IP address or for some other unknown reason.
To me this would mean that short of replacing the router, or resetting it back to default settings and wiping all the devices configured on it there wouldn't be anything I could do to resolve the problem. Given that it's about 4 months since I first had the problem, and all of the other people posting with similar problems since then, it doesn't look like anyone (Yeelight or another user) has thought of a solution either.
The other bulb I bought works fine still though