Re-fixing sagging Tourer headlining

[JohnQBenutti]

It took me ages to track down the cause of serious water leakage into my spare wheel well and under the drivers side carpets. It was the sunroof drains front and rear, of course. After having the carpet up and running a dehumidifier inside the car for a week, it's now decently dry and I can try once more to deal with my sagging headlining problem.
Does anybody have any tips on the best adhesive to use, please? I did read the old thread that recommended using a paint roller to smooth the material back into position so I will try that dodge.
Thanks in advance for any suggestions.
John.