Is the problem getting things wired up within the house or getting a modem to receive a signal at all? I have had to request technicians run a line directly from the box into an outlet at my house before that I then plugged a modem into. From that point forward you're probably looking at something like the power line adapters if you want everything to be wired from room to room without tearing into the walls.

You need to do some investigation in your own. See if your neighbors know where the cables for phone/cable etc go into the units and where they come from. Once you’ve tracked that down look inside and see if you can find where they come in. The cable outside to your unit should be fine even if they remodel ruined what’s inside. From there they should be able to come inside and find where the cables go. As you said they may have tried this already. Worst case someone needs to cut a small hole in the wall where it comes in. Depending on where this is you could just have a jack and out the network equipment close to it.

At this point the suggestion to get a low voltage electrician might be your best bet. You will pay more but they will do a much better job than the cable installer. Have them run coax/cat6 where ever you want things to be.

If they can't find an existing line, why aren't they installing a new one?

When I was a tech and the town house was renovated and I couldn't find the main feed. I would usually go to the neighboring townhouse and ask for permission to take a look inside and look for where their main feed is. 95 percent of the time its at the same location where yours should be to get an idea.