Oh - now I (briefly) understand NATs. And - out of curiosity - why is double NAT a problem? Don't online games (e.g. CS, or Fortnite) work without double NATs? And now, I see why adding another piece of hardware to the chain may be problematic (but since I'm not a gamer, I don't quite understand how it's detrimental, and why port forwarding - which I've never understood the concept of anyway - won't work).
Double NAT causes issues for devices outside your network to communicate with a device in your network.
Usually happens with online games, especially those that have a voice chat feature built-in. It simply won't work.
Fortnite works with double NAT but it's inconsistent and unreliable. Sometimes you'll get errors and won't be able to join a game, sometimes voice chat won't work at all till you restart either Fortnite or your router and so on.
Also, I wonder if I can save on the installation cost and have PTCL use my Nayatel ONT (which is also from Alcatel-Lucent).
Nope. FTTH is way more complicated than DSL in terms of what kind of equipment you can use.
The Alcatel Nayatel ONT is very different from the Alcatel PTCL ONT, so it won't work anyway, but Phase 1 is the only area in Bahria that they're using Alcatel ONTs, which will soon change according to people at the exchange. They'll shift our area to ZTE as well.
Everywhere else they have either FiberHome (some shit brand) or ZTE ONTs. Mostly ZTE though since that's what they're using on areas that have just recently gotten GPON. It'll probably be the ZTE F660.