Dogs are pack animals.
They are not pack animals. A pack is a stable, hardly changing group of individuals who live together as a secure unit. It rarely changes, save for birth and death. Wolves are often pack animals but despite the common ancestor, dogs are not wolves.
We make dogs live like that but feral dogs do not choose to live like that. They form loose and constantly changing social groups of varying size and make up. Who is 'in' the group varies from hour to day to week.
There are a large number of key social differences between dogs and wolves and the instinct to form a pack (or not) is one of the biggies. It's important because, if a group had a tendancy towards an established linear hierarchy (ie an alpha etc) it would need stability in it's membership. Dogs don't have the instinct for that - they do not form linear hierarchies when they group together. There are individuals who want, or are more likely to fight to gain, or maintain, resources but that's a personality thing and does not make them the boss.
OP, your puppy is 5 months old. If feral, this dog may still be sleeping at night with mum and littermates. That's why they are crying. For sure, some dogs cope better with isolation at an earlier age and some learn more quickly that crying is pointless. But that's also a personality thing. A 5 month old crying puppy is not abornomal - it is entirely within the spectrum of normal puppy behaviour. For the most part, feral young dogs will start to migrate away from mum at 6 months+ with some staying quite a few months longer than that, depending on the individual.
At this age, your dog needs reassurance. If they feel safe and confident, you set them up for the next stage of their development: which is to find independence. (Of course, that will come with a whole heap of other challenges and problems!
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I would start each night sleeping downstairs and then quickly migrate up, leaving your pup. When they cried, I would go down and settle them in as calm and boring a manner as possible, then leave them again. Reassure, reassure, reassure. And have patience. Of the dog still cannot be left at night by about 1 year old, then you have an unusually needy dog.
p.s. reassurance will not reward the behaviour, in case you worry about that. Whenever a behaviour is driven by a negative emotion (such as crying when left alone) you focus on changing the emotion first, and the behaviour will follow. You don't focus on changing the behaviour and hope the emotion falls in line. Your dog is scared, so that is the emotion you need to change.
p.p.s. Dog Training on FB is very good. Southend Dog Training is not. Just to add my voice to the others.