I have seen reports, though I've not experienced this myself, that some ISPs who supply the router are turning off 2.4 GHz network, and using 5 GHz only. Others that have seen this may be able to report on it, and I'm only repeating second- (or maybe even third-) hand information.
The model 3700 Express only supports 2.4 GHz, so if your ISP's wireless router turned it off, being "helpful," then that's the cause. Check to see if it's running on your network.
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