You have described a faulty selector switch (known as a commutator at Smeg)
As you have changed the selector switch and it hasn't worked at all, I'd suggest checking the part number on the new switch against the part number on the old switch.
If it's different then it will need to be rewired to suit the new switch.
Smeg can provide this information if you are an engineer and if you purchased the new switch from them. Otherwise, they simply refer you to your supplier.
Check the clock, if set, check the part number on the old switch against the new. (should be printed on the switch).
Let me now what you find, I do have a small selection of technical bulletins on Smeg commutators and once I know the number of the old and new switch, I can check.
Hope this helps
If your oven has a clock, did you set the clock to take it off of Auto operation?