Natural Explorer does not apply to Avernus
Natural explorer works in a natural environment.
You are particularly familiar with one type of natural environment and
are adept at traveling and surviving in such regions. Choose one type
of favored terrain: arctic, coast, desert, forest, grassland,
mountain, or swamp. (SRD, Ranger, p. 36)
Avernus is not a natural terrain — if anything it's supernatural
Geography warps at the whims of the Nine Hells
(locations' names spoiler ahead):
…characters might need to travel 6 miles to get from Fort Knucklebone to Haruman's Hill, and 60 miles to get from Haruman's hill to Fort Knucklebone (page 9 of the hardcover)
and there is an optional rule that affects travel (emphasis is mine):
Avernus's combination of oppressive heat and supernatural malevolence weighs on the bodies and the souls of those who are not evil. A non-evil creature treats normal travel through Avernus as a forced march (p. 79)
Affecting folks based on their alignment is definitely not a property of a natural terrain.
Day and night cycle is absent, so are celestial bodies in the sky (page 76) which I would expect to mess with the "not getting lost" thing.
Avernus features difficult terrain in form of «Bone fields, quicksand, bubbling tar pits, lakes of lava, canyons of wailing souls, and salt flats made from the tears of the damned» (p. 76). Interacting with those is not something to be learned from any single natural terrain. Maybe combining desert with mountains and swamp would cover these to some extent.
It is not natural in terms of foraging either. «Most of the wildlife on Avernus is not edible to mortals, and most sources of water are poisonous or otherwise tainted» (p. 9). Which mechanically translates to:
Wisdom (Survival) checks to forage in Avernus are made with disadvantage (p. 78)
While Avernus certainly had desert-like features it is still a wasteland located in a literal Hell. I think it would be a stretch to say that skills of surviving in a natural desert can be applied to Avernus any more than they can be applied to, say, arctic terrain.