Hello and welcome to the BAEC forum on FT.
Looks like you're trying to book an open jaw trip, outbound leg before 29/10/24 in order to utilise inbound on 5/11/24.
The distance between your arrival and departure airports must be less than the distance of the flight back home.
So if DUB-MIA distance is shorter than MIA-LHR (which it is) then you should be able to book using a 2-4-1 voucher.

I would book the outbound online using the voucher and call up to add the return once you've found inbound availability.