Thanks again.
That is exactly what I saw and this means in fact that you cannot switch from one bank to another with the app. The only thing that you do is to select a preset (but not a bank) and that does not affect what is available from the hardware itself which is only bank 1. This means that, if you want to use specific presets without being connected to an IOS device, you have to override those stored in Bank 1. This is an incredible limitation or a marketing strategy to make you buy a FBV Shortboard MKII which is almost as expensive as the Amplifi 75 !
So, in order to be able to use my FBV Express MKII, I will copy the presets of bank 1 into bank 25 in order to save them and then store 4 of my own tones into Bank 1. This will enable me to footswitch on the fly while playing, without needing to use the app. But this means that I have in fact an amplifier offering 4 presets but not 100 presets.