Dirt in sewer cleanout

I'm having trouble with a plugged sewer line at my house. House is 80 years old and has two cleanouts in the basement, one is 2 inch, the other is 3 inch. I have snaked both of these with a 50 foot Marco Powerpak 60 and pulled out some balled up roots. Drain is still clogged. There is an exterior cleanout that I decided to try next. It had a brass cap on it that I could not unscrew, so I removed it with a cold chisel and hammer. The exterior vertical cleanout pipe is 3 inch. Using a flashlight, I can see 3 feet down it where it is filled with dry dirt. I tried to run the Marco rooter down it, but the dirt was dry and hard packed. I stuck a drain cleaning bladder on the end of a garden hose down there and that forced water out of the cleanouts in my basement. I'm looking for advice on how to get the dirt out of the exterior cleanout so that I can clear my line. Thanks.

Hire a drain company that has a high pressure jetter.
Make sure you close the clean outs in the basement.
Or you might find on at a rental yard.

It will blow a hole through the dirt with a forward spray and then blow it back with rear spray. the rear jets will pull it further into the drain.

Best bet is to hire someone who knows how to use it.

Mr. David,
Thanks for the response, thorough explanation and especially the pictures. I'll update with any progress.

Yes I recommend you hire a company which specialises in blocked drains and carry High Pressure Waterjets, cameras to clean and advise you on the best way to repair since you will have cracked and collapsed pipes no doubt.