Depends if you can get the sides of the split to go back together or in the same plane. If they do, Iโd go with what Ralph said, adding Eternabond on the inside.
If not, then either replace the wheelwell, or do something like Guy suggested. And I would glass it inside and out.
I might be tempted to just Eternabond inside and out at first and see how long it lasts. Could be eternally.
Kip
2015 Skyline Dart 214RB
2018 Silverado Double Cab 4x4
Andersen Hitch