You can order the OEM steel bumper for the Jeep, or is there a different one you want? When I bought mine I signed a paper saying modifications would not alter the warranty. I don’t know about a lease situation though.
As long as you change it back to stock if you decide not to buy it at the end of the lease you should be ok. The new bumpers will not be under warranty obviously and unless they are “proven” to cause a failure of a warranty-covered part swapping them out shouldn’t “void the warranty.” Check your lease contract to be sure.