How can I get my rescue dog to stop pooping in the house?

We rescued a little white puffball of a dog about 2.5 weeks ago. He is about 2 years old according to the vet. We also have another dog, a Shih Tzu, that is about a year old. Up until about 5 days ago, our new rescue dog had been pooping in the backyard just fine. So for two weeks after bringing him home, he was going outside in the back yard without any issues. Now, he seems to be pooping in the house and always by the front door. He does it in the middle of the night when we're not around. We discovered that he'd try and wander around during the day, too, and find a spot to poop in away from us. Since we figured that out, he's been kept on a leash all day long and kept by himself at night and not allowed to wander around the house right now. I take him out several times a day and tell him "go potty" but he still won't poop outside. I even walk him (if its not raining) every day. If I give him even half a second and try non-leashed vigorous play to get his bowels moving, he'll still run away and to his spot. By the time I catch up to him, he's already done his business. He won't play much on his leash, so I don't know what else to do as far as that goes. Suggestions and/or ideas are appreciated to get him to go and do his business outside in the backyard. Our other dog has his own designated spot where he does his business. I should mention that we have also tried to give our new dog his own spot to relieve himself. He does pee without any issues. It's just the poop that we can't figure out. Thanks so much!!I should also mention that he DOES let us know when he needs to pee - he'll scratch at the back door to let us know. We just can't figure out how to get him to stop pooping by the FRONT door instead of outside. Oh, and he does have his own designated spot outside which he uses to pee and he used to use it to poop until recently. Not sure what changed.Oh, and I'm taking him out every 30 minutes or so for him to relieve himself. He still won't go despite how often I take him outside.

Have a designated spot for your dog to go to the bathroom.

you have to crate train him to not poop in the housewe did this with my dog when he was younger whenever we left the house....and he only pooped once in his crate, and never did it again, and then when we came home we would walk him and he would go to the bathroom outside, and then we would have wee wee pads in our house too to make him learn he cant go on the carpet, and tht helped too,but if you dont let him out often even if he is potty trained hes gonna go in the house if he needs to go.

what i do is vary carefully watch him/her and when he/she is about to pee or poop then real quick get him/her out side fast so he/she can pee or poop.

You need to crate him at night. He will stop.