This is a combination of issues. Not just lack of food, but also severe sunburn.
Citrus is a closed canopy tree, and your canopy is very open and there are burns all over the bark. Need to find a way to shade/filter the direct sun it until it makes new coverings.
Get some foliars and feed it a rotation weekly until it recovers, and possibly change your ground feeding to something which won't burn it meanwhile, such as Osmocote Plus and Premium worm castings, etc.
Cut off those dead, damaged and diseased branches asap.