Raptor what setting is s20 was doing that? I used s20 and never knew there is such option. And same goes for any other device & os including p8 with gos, of o am not missing something of course.
Some firewall app like netguard allows to set almost any app to use either WiFi or cellular or both, including system apps. saying almost "any" cause some system apps are united into groups, that have such setting be for all that group, without an option to set each of apps in it to connect thru different channels. But that is limited to some system apps only.
Using such method requires using the VPN slot, thus it makes use of a VPN on the device not possible, still can have a VPN on the router so all wifif-connected apps are routed thru VPN. But not thrones that connect via cellular.