I hesitate to recommend any vet or person who isn't an experienced dobe owner, because there are a lot of vets and staff out there who really don't have a clue how to post. They're not taught that in school, and have an awful else to learn too; posting dobe ears is down there on the list of priorities to be familiar with.
Have you checked our thread: https://www.dobermantalk.com/ear-cro...post-here.html
It's an older thread, and may be a bit out of date, but you may see someone close by who can help you out.
You could also try calling your local (or as close by as you can get) doberman chapter club...there are likely some folks there who would be willing to help you out, or point you to someone who can.
I would encourage you to keep trying. Posting is one of those things which seems very difficult at first, but once you get the hang of it, it goes pretty smoothly. And it's so much easier to be able to deal with those little emergencies (posts popping out of the ears, getting filthy or wet, evaluating whether possible sore spots are starting) right away than having to wait for an appointment to have someone else take a look.
I don't know what method you're trying to use, but we have a tutorial here that we recommend to a lot of people: https://www.dobermantalk.com/ear-cro...acker-rod.html
We also have another thread which has a lot about how to post if you keep having trouble with the posts popping out. Believe me, it is a very common problem.
Read through the entire thread particularly post #9, 13, 15 and 17. Those particular posts deal with how to post to get the posts to stay in place.. https://www.dobermantalk.com/ear-cro...s-posting.html