Reverse osmosis water saddle doesn't fit waste pipe
I ran into a similar situation and not being able to locate the correct sized part (saddle) I successfully installed a hose barb fitting

Looking at your photo I can't easily find an acceptable location for a threaded barb fitting. Ideally it should be oriented vertically and on top of any pipe section. And obviously before the trap.
If it's possible to add a short extension of pipe to the connection where the flexible hose is attached so as to allow an attachment point that would facilitate connecting the new fitting.
Installation is a simple matter of drilling a slightly undersized hole and forcefully turning the fitting into it after wrapping the threads with Teflon tape.