Ive cleared a couple of clogs. Its a spooky thing to open the lid of a septic tank when sewage is backed up into the house 5 or 6 feet above. Either the inlet is plugged ( usually a big wad of tp at the inlet baffle) or the outlet is plugged and if it's the outlet then you need to be ready for a fountain of sewage when you lift that lid.
When I put a riser on my tank the first time I pumped it I also put a cleanout riser at the inlet pipe so I could clear any clogs that happened there. Since my tank is buried about 4.5 feet down it was worth the effort and cost to put the risers on. I've cleared the inlet clog once and was so glad I had the cleanout riser.