Hard to say since both are new.
You can pull the toilet and set it on blocks in the yard, try flushing it there.
If the line is new, is there a test plug in the line, or test cookie?
Did you put the wax on the bowl? or set it on the flange during the installation. Sometimes the wax can fall and drop in the wrong place.
Most plumbers set the wax on the flange, and drop the bowl down onto it.