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That sounds like a standard problem that will give way to a few routine measures. But, have you switched the routers yet, and is that where the problem is? or is the problem that the printer is Offline and this is AFTER the router switch has already been done? (that last is standard.)
Let me know, please. And I will help and advise you further. Thanks.
(I presume your Brother printer is supposed to be on a Wireless connection.)
Please tell me whether your computer, that prints to this Brother printer, is a Windows PC, or an Apple computer. And what version of Windows, or MAC OS X, it has on it.