Is it pomodoro that doesn't work or just timers in general? One of the more heavy handed approaches I've taken in the past is installing a parental control app on my phone (if you use Android, there's a really good FOSS one available on F-Droid), and restricted access to all websites and apps aside from calling and texting during work hours. I obviously know the password to disable these, but the extra time spent punching that in is enough to steer myself back in the right direction.
I know you said timers don't work but I'll still suggest using them, just in a more loose fashion. Momentum is huge with executive dysfunction, getting the ball rolling in the correct direction is important. So instead of holding yourself to a hard and fast 20-5-15 Pomo, try "I have to work for the next 5 minutes, after the timer is up, I can take a break, or I can keep going if I feel up to it". That 5 minutes is adjustable, if 5 is too long, don't feel guilty about lower it to 4, or 3, or even sub 1 minute. If you aren't picking up any momentum after few tries, stop and ask yourself what needs could you meet before trying again (Hungry? Tired? Craving a burst of entertainment?).
Lastly, lets address likely why some things only work for you for a few weeks. Novelty, is an incredibly powerful motivator for the adhd brain. Simply something being new can be enough to make it hold your attention. As soon as it's not new, it no longer works, because sadly habits aren't very "sticky" for the adhd brain either. BUT, you can still use this to your advantage. All you need is multiple systems to get yourself in gear, and work them on a cycle. Use one method until you feel the drive burning out, then switch to a new one. On a constant cycle you can keep the novelty rolling and keep yourself involved in organizational aids.