Sounds like the guy is a true hack. You never beat a hole in a septic tank, and unless its 40 years old has a ton of wire or rebar in it.
You MIGHT be able to cut a hole in the top if it gets you into the first chanber, you must be past the baffle, location of which is unknown, usually. I assume you have access to the second chamber..... then you can find the baffle.
Then you cover it with a new casting or 3/8" steel plate - plastic and junk? Call a dedicated septic pumper, not the rotorooter dingbat.
Cut the slab, reline the hole with 1/2" foam spacers, re form the edge, and cast it with a recessed handle or hook. I have the same situation in my walkway, and I bricked [sand set] over such a lid, and set a large brass cap with septic embossed into it.
If bothering to cut a hole, might as well be in the patio.