Well are you sure you can’t use any public companies as benchmarks? It doesn’t need to a direct competitor, just a similar company.
assuming there really are no public companies that are similar you would just use a DCF. For your beta maybe look a bit broader to get a sense of what other telecoms are, even if they are not similar enough to use for multiples. Or just make your own judgement as to if it’s more or less correlated to the market.
depending on how asset heavy your company is you could also do an NAV as like a liquidation value.