because you have a combined network you'll need to do option 2 ww mentioned. in a combined network there are hidden node group names appended to the networks: - appliance, -switch, -wireless, and so on.
so, make a copy of your splash page then edit the continue.html file in there. should be a super quick fix.
first create a new splash page and you can choose copy from your current "modern" page.
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Next, click the continue.html file and then the edit tab. Find the line with WELCOME and type whatever you want between the <h1></h1> tags.
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