Lagoon Nebula is really really big and I think than 90x is simply to much power to frame it well.
Sky Safari lists the size of this nebula as 90 x 40 arc minutes, so using 90x means that it simply does not fit into the field of view.
Fitting into the field of view is important because the nebula itself is only a bright knot that sits on a background that itself is glowing so to get the contrast necessary to make this nebula stand out from the background glow, I think a field of view of at least a degree is going to be beneficial.
This target is no better in a big scope than in a small scope. Exit pupil being larger helps. A light pollution filter might also help. I recommend a degree or larger true field to see it, and to see it well, 40x in an 8" scope under dark skies will provide a decent view.