If you have any mods enabled, it will disable your ability to get achievements. If you have a city that you started using mods on, then that city is "dead". Your city must never have the taste of mods if you want achievements. I suggest having a separate city for achievements and naming it something so that you know to disable mods when playing on it.
If you can make a big enough city, then this one is super simple.
Monuments unlock when you have a population of 90,000. So, you just have to keep playing until you can build a city that is 90,000 or larger. You'll get it as soon as the window pops up telling you what you've unlocked.