20 Tiglath-Pilesera king of Assyria came to him, but he gave him trouble instead of help.

Read 2 Chronicles 28:20 Using Other Translations

And Tilgathpilneser king of Assyria came unto him, and distressed him, but strengthened him not.
So Tiglath-pileser king of Assyria came against him and afflicted him instead of strengthening him.
So when King Tiglath-pileser of Assyria arrived, he attacked Ahaz instead of helping him.