you need an exterior pva wood glue but looking at the photo you have damage to your board.matchboard are placed one after another during fixing ie tongue and groove,so to take a middle section out without damage is impossible. The remedy is to take the old board and split the back part of the groove completely out with a stanley knife,thus forming a rebate apply glue to tongue and rebate and slip back into place.
masking tape will work but the more pressure the better the bond,try wedging it with 2x1 from shower tray to board for a few hours
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