There appears to be more than one "fix" attempted, here. The amber substance looks like a resin, either polyester or epoxy. the other grey colored material around the neck looks like hysol. If the grey material is somewhat rubbery, then it might be ProSeal.

The fix is to remove any loose material and rough sand the area to be repaired.
Flush with denatured alcohol. Blow dry with an air nozzle.

Use fiberglass mat and West System epoxy to cover the damaged area. I find mat works better than cloth when working blind in a tank. You may have to work with the tank suspended over your head to prevent the resin from dribbling away from the parch when you seal the inside. Let cure and then flip the tank right side up to seal from the outside. Allow West System to cure at least 24 hours before filling the tank.

West Sytem is an epoxy and will adhere to polyester resins. It is very effective in sealing fiberglass tanks. The hard part is getting the area inside the tank cleaned and prepped.

It may take some creativity to make some hook shaped scrapers and sanders to reach and prep the area. Make scrapers from 1/2 inch by 1/8 inch flat steel (Home Depot). Hammer into a "J" shape and grind a sharp edge on the end. Shave and scrape the area to be bonded. With a little perseverance you can reach almost any part inside a tank. If you can't get all the old material off, just overlap with the new stuff by two inches or so. Do the same on the outside.