OP said slack. DM is as simple as doing "/msg @joe.blow I'm blocked, can you please help me?". If no response, go to @joe.blow's colleagues. If still no response, go to their boss. If still no response, go to their bosses's boss. Repeat until you run out of people to escalate to, at which point you find another job.
If they are willing to pay a blocked remote worker for sitting on their ass doing nothing, why should the worker rock the boat so much that they burn all bridges at the company and finally have to get a different job? Remote jobs seem somewhat hard to come by and can be a sweet deal.
The company needs to take at least partial blame here.
No need for worker to do that. But that's then their conscious decision and they shouldn't express their frustrations over it. On the other hand, if they want to make a progress, they should escalate appropriately.
Note that I am not convinced that this is the case here - I simply don't have enough information to make the judgement.
So someone wanting the system they are a part of to work appropriately without them having to be an excessively aggressive asshat about it needs to swallow their frustration entirely and not even vent about it on an online forum?
Even!more than that, at some point you start including "You are paying me $xxx/day to do nothing productive because I'm unable to get 15 minutes of time or even acknowledgement of requests from persons X, Y and Z."
My other thought was to file blocking bug/defect issues on the relevant software because obviously if it can't build in the first place it's certainly not passing QA.
Some of this may get to the level of passive-aggressive or aggressive-aggressive bridge burning.
