Finding sewer line in yard?

You probably don't need to break things and put in a new cleanout...you need a properly sized, professional snake which should be able to be extended nearly any length to clean out the blockage.

If you used a small diameter snake, or didn't go far enough, you may never get it cleaned out.

If you dig up around the cleanout you did use, you should be able to see which way the line goes as a start.

I would suggest calling in a pro to take care of your problem. They will have the proper equipment, Knowledge, and resources to take care of your problem and get the drains working again. In most cases within a few hours of you placing the call!

DIY snakes are seldom effective. They are too lightweight and too short to do the job. Often they just push the clog deeper and tighter, or they just push through the clog without clearing it. A professional auger is not something that you should try to use even if you can get one. These are serious tools and have no hesitation in injuring a novice. A clogged drain should be cleared all the way to the street.

since you've already dug up your yard, dig down at your clean out and start chasing the pipe. this way you know exactly where it goes and can add clean outs as you desire.

or, you can have someone camera and locate the whole thing. this would also show you the bad spots.

The line would have to be cleared first before the camera would show you anything except location.

As I stated above you need a pro!

A pro would have:

A proper size snake for the job
A camera if needed
A jetter if needed

A pro would be able to:

Know how to snake the line clear if possible
Locate where additional cleanouts are needed or, where they are
Locate any defect in the line that requires repairs and make necessary repairs

In short a pro will Git-R-Done!