dumpsters[model] is looking for something at index 218085040 in the table which doesn’t exist. You need to iterate over dumpsters to check if the model is in there:

function inTable(tab, val)
    for index, value in ipairs(tab) do
        if value == val then
            return true
        end
    end

    return false
end

Citizen.CreateThread(function()
    local objects = GetGamePool('CObject')

    for _, entity in ipairs(objects) do
        local model = GetEntityModel(entity)

        if inTable(dumpsters, model) then
            print(model)
        end
    end
end)