You have no back door. Every toilet trip is shoes, keys, a lift or 3 flights of stairs, and somebody on the landing.
Owners in flats assume the flat is the problem. Usually the flat has removed the accidental outlets, so the deliberate ones have to carry more.
Sniffing and chewing beat distance walked
Scatter half of one meal into a towel, a mat or a cardboard box, and let your dog work through it.
Walk shorter and slower, letting your dog pick the lampposts and hedges. A 20 minute sniffing walk can settle a dog that a fast hour does not.
Give one long chew a day, on a safe and appropriately sized item, in the same spot each time.
Hide 5 pieces of food around the living room while your dog waits in the hall, then release them to find it.
Relaxation is trained as a behaviour in its own right, in short repeatable sets below threshold. It does not arrive as a side effect of tiredness.
The hallway and the stairwell are your training ground
You have a quiet, dull, semi-public corridor 3 feet from your front door. For most of this work it beats the park.
Do 2 minutes of loose leash walking up and down the corridor, paying every step beside you, before you go anywhere.
Practise sitting at the lift door or the top of the stairs while nothing happens, and feed for stillness.
Open the front door, feed your dog for staying inside, and close it again. 6 reps, with no walk attached.
Practise passing a neighbour at whatever distance your dog can manage, on a short leash, with food arriving as they go by.
Tell one neighbour plainly what you are working on. A neighbour who knows is easier to live beside than one who is guessing.
Settling with the building going on around you
Your dog hears the lift, the door buzzer and the flat above at all hours. They cannot learn to ignore any of it while they are rehearsing barking at it.
Move the bed away from the hall wall and the front door, where fewer sounds arrive.
Run steady background sound at ordinary volume through the busy hours, to blunt the edge of individual noises.
Feed your dog on their mat each time the lift runs and nothing else happens, for a few days.
Practise settle in 3 minute sets at a quiet hour first, and only then during the evening traffic.
Working below the threshold at which your dog reacts is the condition for learning. In a flat that mostly means choosing your hour.
6am, and no back door
Keep a bag by the door with a leash, a coat, keys and food in it, so the first trip out takes 30 seconds.
Take your dog out on a collar and leash and ask for nothing. This is a toilet trip, not a walk or a session.
Stand still outside rather than walking, then bring your dog straight back in and back to bed.
Feed a chew in the safe spot when you get back in, so the hour after is quiet for the neighbours.
Behaviour change is built from management plus reinforcement. In a flat the management half is mostly a packed bag and a chew at the right hour.
Safety note
See your veterinarian if your dog starts asking to go out much more often, has accidents indoors after being reliable, or wakes in distress. Sudden behaviour change warrants a veterinary examination, and pain often presents as a change in behaviour rather than as obvious lameness.
Today's version, in one line: scatter half a meal into a towel, and do 2 minutes of corridor walking before the next outing.