How can I glue behind my tub surround without removing it?

Tub surround is from an old installation and has to be at least 14 years old. Which is as long as we have owned the house. While getting ready to re-caulk the tub we found that the tub surround is pulling away from the drywall behind it. The highest is over a foot from the tub up the wall. That is from the middle of the tub/wall horizontally. Near the edges it appears fine.

We can pull on the surround enough to be able to see a little behind. The drywall behind looks great. I see no water damage. We plan on renovating the bathroom within the next two years… but not now. Removing the tub surround partition that needs the most significant gluing seems very risky as it feels like it might crack the tub surround if we pull on it too much and that would make for a way worse situation

I wanted to see if there was a way to get enough glue behind to get some minor adhesion. I attached foot and half length of 1/4 inch vinyl hose to a pl premium tube with pull ties. Tried to see if I could force product through. That failed after about 6 inches as the force pushed the tube off.

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Worst case is we are going to try and use a small putty knife to get the bottom couple of inches covered and hope for the best.