When I was using Ubuntu 16.04, duplex printing worked like a charm with my Brother DCP-J4120DW. However, I switched to Kubuntu 18.04. This makes Okular the default document viewer and also my document viewer of choice. On Ubuntu, I mainly used Ubuntu's default PDF viewer Evince and duplex printing always worked after I configured the printer that way in Ubuntu's printer settings.
I set the option "Two-Sided" in Kubuntu's printer settings to "Short-Edge Binding". However, this seems to have had little effect as Okular prints non-duplex by default. If I change the option in Okular to short-edge-bound duplex printing, it works. But it doesn't remember this choice. I checked the default printing options of Evince and Evolution and they say that two-sided printing is "Not available".
How can I make applications print two-sided by default?
I only care about this one printer as I only use one.