Sounds like you have a 5th wheel.....toilet line drops down has a 45* then drops again with another 45* then goes to tank.
As suggested.......
Go to hardware store (Home Depot/Lowes/ACE) and buy some 3/4" BX electric cable. Unravel one end of metal sheathing to make a cutting blade and bend wires out.
Turn water source off......run cable down thru toilet twisting it as you go pushing and pulling cable back & forth.
With many attempts, patience and git-r-done attitude.
Otherwise.....call around for a septic tank vacuum truck and have them suck the toilet and drain line all the way down into tank while flushing down thru toilet with water.
Something got dropped down in toilet.......hair brush, wash cloth, bathroom items etc. or large poop pyramid built up---which can dry rock solid.