Hurray, UPS has disgorged replacement modem on date $DSL_provider alleged it would. Perversely, old modem has been behaving itself, perhaps because it's not the wee hours going into a weekend (machines save their breakdowns for the maximum stress of "an hour when you can't possibly do anything about it", I am firmly convinced -- honestly, 90% of the issues happened on Friday-ish for some reason), so I will hold off disturbing the equilibrium of the system by swapping the new one in so long as things are Working For Now, but I feel much relieved to have a spare in my possession. And now I can finally go to bed...
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