Are the interpolation modes greyed out like this?

Then it could be that you ‘locked’ the channels (see lock icon at the right side of the channels). There is also the wrench icon that when not enabled will not use curve modifiers….
Also the interpolation only changes between selected keys, so you must have keys selected (not just the curve), but I guess you know that already…
I am just guessing here; don’t know of any way to disable interpolation or modifiers ‘permanently’ through Preferences or so.
If this is not it, you should ask an expert like @waylow .