@martin2 did you get this resolved? I am having a similar problem on the SE.
https://forum.flylitchi.com/t/litchi-only-connecting-at-2-4-ghz-on-mini-se-maybe/2419
When I manually select a channel in the FLY app, it is reflected in Litchi, but not the Frequency.
As the manual channel I selected is outside of the 2.4 GHz channels, I suspect this is a bug in the Mini SE version and it is in fact using 5.8 GHz in Litchi