I use a iOS VPN app called Speedify, that simultaneously bonds wifi/cellular channels into a single channel, ostensibly for speed sake. It seems to handle situations where either wifi or cellular is flakey, and even has a redundant mode, where all traffic is sent via both channels simultaneously. Redundant mode is not fast, but it is rock solid reliable. I use it if I want to make sure I don't drop a VOIP call.
Because of Apple's silly disabled-but-not-really wifi and bluetooth, my wifi keeps switching to on, seemingly randomly, but probably whenever it loses and then reacquires my home wifi.
As a result I have to keep turning wifi off in settings, which is a bit too rigorous, or I have to keep remembering to turn off wifi in the bedroom.
I can't help but feel there's a use-case myopia at Apple that, for whatever reason, is starting to become problematic. For me, at least, and I think I'm pretty much their ideal customer (aside from not having a wife and kids).