Alex,mathematically, "infinite wavelength" is perfectly correct thing to say.
Especially if we start with some (short) wavelength which becomes longer as expansion progresses.
Photons arriving from the exact edge of the visible Universe will be DC...
Photons that arrive from a bit closer distance will have finite (but very long) wavelength.
Are you familiar with math term called "limes"? (or limit, I believe this is English term)
You have to get accustomed to mathematical way of thinking
BTW, this why we are talking about the "edge of the visible Universe" because we can not see anything at or beyond this (mathematical) boundary.