It is a bit of a mystery why Roon offers those MQA configuration options for a device they themselves have certified as Roon Ready and that is well known to have no MQA capabilities whatsoever (so changing the settings has absolutely no effect). See posts 26 onwards in the same thread.
Since Phantoms don’t support MQA there is no circumventing the 24/48 downsampling, even with MQA encoded material. The only way to render higher resolution material on a current Phantom is with Direct Optical Mode.