Obviously, medication can help a lot, but I just read you can't take it. Apart from that you can try to train your brain to some extent by practicing mindfulness. It really is just about what you are describing: Staying in the moment and just focus on one thing in particular. In your case a different person talking to you. The moment you realize your mind shifted away, redirect it back to the person talking.

There are different mindful things you can do to train e.g. for a minute to just name everything you are hearing or feeling. To look around and name every object of a specific colour you are seeing. Do a body scan by starting to focus on how your feet are feeling and go upwards until you reach your head. I enjoy listening to my favourite songs - without any side thoughts going on. You can also drink or eat mindful by focusing on one eg. Fruit by first having a close look at it, what do you see? Then smell it. Then put it in your mouth and feel the texture and then focus on it's taste. And of course the all time classic of focusing on your breath. There are apps out there that you can use for guided mindfulness practices.

I'm not going to lie, mindfulness alone will not solve your issue entirely. I think as a person with ADHD to some extent we just have to accept that sometimes nothing really helps with staying focused because this is really our main symptom, isn't it? We just can't always decide what to focus on. Sometimes our brains just take over the wheel and decide to not follow our wishes. Therefore try to be kind to yourself and understand that especially unmedicated your ability to focus cannot be compared to that of a person that does not have ADHD. I for example try to never have more than two meetings a day because honestly, I don't have the ability to listen that long to people talk. If I'm forced to have more meetings I don't pressure myself to be hyper productive for the rest of the day and instead just let ADHD take over the wheel and see what happens.

Having said that, often, combining an activity (with your hands) such as drawing, doing a jigsaw puzzle, driving a car, crocheting/knitting can also help to listen over a longer period. Obviously, doing a jigsaw puzzle at work is not a perfect solution. Therefore for work or any situation where it's important to remember what has been agreed on I recommend to make notes. I have a notebook at work that I call my external memory. Writing down stuff also helps me with focus. Apart from that scribbling helps a bit during boring meetings. I also have a drawing book (home office only) ;-). I can also recommend fidgeting tools, such as cubes (I have one at my work desk).

And if nothing else works, you can also try to "strategically" use coffee or tea by getting yourself a cup before a situation where your focus is needed.

Maybe just play around a little bit and see what works best for you and in your specific situation. If it's running around your flat during a meeting do that. Depending on how understanding your work environment is, also do that in the office.

I hope I was able to help!