When we took Bonnie at 5 months to get help on BO training for US and her, at the place we got help with a big strong GSD rescue two dogs back, they recommended
1. at least an hour a day of exercise- walk,
and
2. another period of play/interaction/training mixed in of 30 min.
And
3. we found Bonnie had another active period we filled with play and chew toys and she conked after...boom like a tired toddler lights out.
If you can maintain a regular schedule you'll find the rest periods in between become natural, and you can use the crate to reinforce that, inc open door covered with a blanket- its their den where they go to switch off the first ever trainer told me- a dog psychologist and breeder, and behavioral trainer 30 years ago. Its not punishment, its the refuge.
If you cant walk, jog, bike or hike trails at least an hour a day, hire help or think hard on what you want...a PITA or a happy dog.
A tired dog is a happy dog..
This is the advice from the K9 trainer we used: 40 years of experience training, boarding LEO k9s, inc dobes back in the day and now training GSDs, Mals, and other working dogs for several LEAs and Border Patrol. PM and I'll send a link.
Last time I was there they were training and had some rotties and field labs for executive family type clients, but I dont want to misrepresent or appear to be selling...
just an example of where this advice came from...not some rando anon old fart on the innertubes: me.
Its possible this is what you may need help with if this dog is out of your league despite many dobes in past...ask the working line owners and trainers here for other names of trainers in your area who know dobes or how to find one...
One big tip was go to different places, esp with sniffies and you will get twice the bang -an hour of brainwork is equal to an hour of play.
If you have had these dogs before you know this is a LOT but its worth it. You just have one on the "needs more end of spectrum.
Remember what trainers say- dogs go thru periods of "remission" or critical learning periods...they forget or regress a bit...be patient go back to something easier they know and praise and work that.
search archives here or elsewhere, you'll see its not your fault. You just have to back up to the basics, and retrain in effect, until they get back on track.
If the basics WERE weak, then just go back to the beginning. Be creative, try different things. Not every dog learns the same so look for what does turn on the lightbulb and use that.
These dogs WILL train you, if you are not consistent. Try not to let frustration rule you, take a deep breath, find the center of calm...And mostly HAVE FUN!
Its never perfect, just keep at it and suddenly at 2 years in girls and (I read 3 in goofy big boys) that their brains come in and you have the best dog ever!
Amiright? Experts here please give thumbs up and correct if I am wrong...
I am not a trainer so take with handful of salt.
Only 2 girls for me so far but this has worked with both and a handful of other strong stubborn breeds: listen to the dog in front of you, apply kindness, love, patience
Clear communication and boundaries, consistency...treats for incremental inprivements...
And dont forget to give yourself affirmation and rewards too for all your hard work!