[Help]: Advice/tips/guidance on house training 6 Mo. old Puppy

My husband and I rescued a Lab mix puppy, who is now ~6Mo. old, from a shelter about 4 months ago and we are still having issues with house training. We have read a lot of books and articles on the subject and have tried just about everything we can think of. We go out on a set schedule, he gets the super tasty treats only when he goes potty outside along with lots of praise. He still doesn't seem to connect the dots that he is supposed to go outside. I have cleaned and shampooed the carpet with special enzymatic carpet cleaner to get rid of the smell to discourage him from going potty there, but he still does. Some days he will need to pee every 30mins, and some days he can go 2hrs without needing to pee. He is crate trained and is in his crate when we aren't watching him closely. He gives hardly any warning about needing to go potty. One minute, he could be running around and playing and then suddenly he just stops and goes without giving the typical "I need to potty" signs. Could he be peeing inside because of boredom or for attention? Is there something we are doing wrong? What else can we be doing to help him understand? It has become really frustrating and I'm not sure what else to do.