Maintenance Page using V11

First thing, "[LB::Customer_pool]" isn't a thing. Use "[LB::server pool]" as described in the previous iRule example. This will expand to the currently configured pool when the event is triggered. You can optionally specify a literal pool name:

if { [active_members "Customer_pool"] == 0 }

Second thing, you're still trying to switch on the HTTP::uri, but you don't need this since you're not presenting anything more than a single HTML page. Did you try the previous iRule example?