I've had luck with Natures-Miracle for Cats but you can also use some of the enzyme products made for removing odor from kennels. You're going to need to saturate the area so don't just lightly spray it around the porch. I would buy the large containers then use a garden sprayer to apply it. You may have to do this several times......letting it dry in between each treatment. Also, don't forget to treat the walls at least 12 inches up from the floor. Here's info on why you NEED to use an enzyme cleaner.

When cat urine has permeated into anything remotely porous, it’s going to be an expensive problem where everything has to be ripped out and replaced - it might be cheaper to burn the house down and start fresh with a new build (I’m only half joking here when I say burn the house down to save money! But dog pee smell is easier to remedy than cat pee, so if I had to choose one to deal with I’d pick the dog pee 100%). Is there an exhaust fan in that bathroom? Like @kelli w, I’m wondering if it’s some kind of mold or bacterial growth. If your bathroom has no ventilation, things like mold/bacteria are definitely going to proliferate on any surface with the right combination of conditions that allow them to do so. Since it’s less costly to go without cleaning the shower and see if the smell worsens; if it does you have a starting point from which to plan a course of actions, which should definitely include putting in an exhaust fan if there’s not one in there already, but it’s only able to help things if it’s actually used during showers and run for a good while afterwards so it can do it’s job completely.

Complain to the landlord. Tell them that you are able to smell the urine and are worried that this is a HEALTH HAZARD! Ask them to FIX THE ISSUE. Put these requests in writing. Do it 3-4 times. If you aren't getting anywhere, you can then contact your local Health Officer and ask them what THEY can do to find out if you are living with a HEALTH VIOLATION. Then go ahead and let the landlord know you have contacted the local Health Agency and will be expecting an inspector to visit you shortly. See what comes of that. As soon as you have a report that says the landlord must fix it BEFORE humans are allowed to live IN the suite you are out of the contract (no penalty to you because the landlord did NOT ensure the suite was ready for human habitation). Start with telling the landlord you have an issue and that you are worried it is a HEALTH HAZARD and that you are afraid the local Health Board may have to get involved. That should move things along QUICKLY. Landlords get FINED if they are in violation of local health codes. And yes, old urine can cause breathing issues and long-term pulmonary concerns. If you can smell it, you are breathing it in. If you have asthma, COPD or compromised immune system this could be a big deal for you.

"Given the high moisture level, I would guess, yes, during the 3 month closing period" I would think that your real estate agent should have suggested this, but have you contacted a real estate attorney to inquire about suing the seller for the cost of all repairs? If you had a signed contract to purchase I would think the seller would have some sort of legal obligation to keep the property in the same, if not better, condition that it was in when you signed the purchase contract. To allow that much dog urine seems willful and deliberate. I'm really sorry you're going through this.