For The Last 3 Years We Have Had Problems In a New House With a Toilet In The Guest Room, Getting Clogged, and Not Flushing Right. We Hired a Plumber to Take Off The Toilet And Inspect It, and Drains, Everything was OK.

I Just Went on The Roof, And Tried to Snake The Vent For This Toilet - About 3 Feet Down, There is a Black (Looks Like ABS) Plug, Blocking The Vent, With Glossy (looks Like Glue) Lines Around This Plug, Which is at a 45 Degree Angle in The Vent Pipe. Attempts to Dislodge It Were Not Successful, as The Entire Vent Pipe Pushes Down - When I Attempted to Dislodge The Plug (Thinks It's Glued)

I Was Going To Drill All Around The Plug To Get it To Dislodge It, But Does That Mean The Plug Will Just Fall Further Down The Vent pipe and Clog It for Good ?

Can I Just Drill a Series of Holes in The Plug, To Allow It to Vent ?

What's The FIX Here ? Thanks

i would drill a hole in the plastic plug and insert a wall anchor on a long piece of threaded rod.
this will keep it from falling down. then proceed to drill holes all around it 'till you can pull it up and out.

Thats a Good Idea, Since The Plug is at a 45 Degree Angle, Any Idea How I Keep The 4 Foot Drill Bit From "WALKING" as I Try and Drill ?

if the pipe sticks above the roof 18 inches or so, you could cut it down a little so you could get away with a shorter bit.
you can easily glue a coupling back onto the cut off pipe.
a brad point bit is my only guess that may work ??

Is the attic accessible?

This whole scenario seems strange. An abs plug is glued into the inside of a pipe at a 45-degree angle, 36” deep down the stack vent at the roof?

You 100% sure you’re not just looking down into the pipe and observing the inside of an elbow bend or something?

Palm my thoughts exactly. A forgotten test cap wouldn't be 3' in nor glued at a 45* angle.

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Not a plumber by trade but a fierce DIYer

Regardless of what it is, or why it is there, a "clogged vent" will NOT prevent the toilet from flushing and in some cases would INCREASE the power of the flush and/or improve drainage.