From my experience if you use an auger with a spear shaped end it will punch a hole through and the pipe may stay clear for a month or two.
A cutting auger, if it's the right size for the pipe, should do a pretty good job of cutting the roots and you might be okay for 6 months or a year. But, if the pipe has a seam open and there is a tree nearby, the roots are going to come back.
You might have to bite the bullet and dig that pipe up and repair-replace.

i assume gap is really just a small seam where pipe sections come together. house was built with septic/well system in backyard. 5 yrs later we got city water/sewer in front yard. we moved in after the work. i assume the line out to street was dug after house was built. so a septic/well was paid for by 1st owner. than city water/sewer was put in and someone got to pay for it. now i get to pay to have main line dug up? its only a 1975 house. i can see digging a new sewer line if its a house built in 1903.

"now i get to pay to have main line dug up? its only a 1975 house. "

What kind of line is their?

Once tree roots get in they WILL be back.

The tree has had more than 30 years to find the line.

The auger end of a power snake, or a manual one, can screw its way into the roots and you may be then able to yank out the snake and the roots it has snared, if the roots are fine enough that they don't hold onto the snake without tearing loose. Do you feel lucky?
I've used this method for annual preventive maintenance but only after RR ran a cutter through the first time.

could see root plug with camera. camera was upside down as it got close so roots were poking up from bottom. looked odd. than we pushed thru root plug. weird view. we were moving closer on tv till we hit plug. than we kept pushing cable in but plug did not appear to move. we were pushing plug. right out to street. maybe 50ft or so. pulled camera back and almost all root fringe was gone. decided to rent cutter and had no drain issues for 1 yr. been plugging up again the last year.

Either replace the line or do repeated clearing.