Can you put a child fence and gate around the outside of the front door, so you have a gate on both sides of the front door.
Something like this ...
55517 Fire screen 300cm FIREGUARD Hearth Gate Child Fire Safety Guard fireplace | eBay
When training ours, we put a ferret line on them, that could be grabbed, stood on, if needed. It's lighter than a lead.
Don't give up on the training, it might just click in. When training, run away from your dog, if they look as if they are going to run off. Call him, sounding excited. Praise when he comes.
We had the same issues with some guests. Years ago, we had a GS pup, and my hubby's friends stayed over. Treated me like a nut job, because I kept asking them not to leave the front door open...ie I have young children and a pup! They knew better and wouldn't listen. The husband walked out, door left open, I run to the door in time to see my pup under the post vans front wheels, and to hear the screech of brakes being slammed on. Luckily the dog lay flat in fear and was between the two front tires. The postman nearly had a heart attack, poor man. (Those guests were never invited back by me, as they still acted like they'd done nothing wrong, and I was just a worry wart)