Isle of Armor Diglett locations: Insular Sea
The Insular Sea is just to the north of the Stepping-Stone Sea, and has six Diglett to find. As you'll see though, that only involves going to four locations.
Go to the larger, circular island in the Insular Sea and the first Diglett is in these red flowers by the berry tree.
Then on the eastern side you've got this Diglett in the sand.
Followed by this one on the north-west side.
Get your bike out and go to this smaller island, where you'll see two Diglett in the ground. Interact with them and voila! There's an Alolan Dugtrio hiding their too, which counts for three Diglett... but wouldn't it be five, since Dugtrio consists of three? Come on Game Freak.