Since Wizards.com is something you say isn’t available: Drivethru.rpg is a good website, to get PDF versions of various rpg rule books. Along with other more small sources, like examples of “quick npcs” (which could perhaps become PCs if you need pregens) and “how to improvise”. Examples of Adventure Leagues sessions to run too if you want something, ready to go.
The official 5e DnD player handbooks themselves specifically, are however not available on Drivethru.rpg after having a look. Although, they do have a whole “Core Rulebooks” section, with plenty of others (like pathfinder, older edition dnd, fate core, dungeon world, cyberpunk, etc). If any alternatives could be an option for you.
But essentially, with it in mind that your internet is fairly limited: If you look for one which suits you and what you want to play, you should be able to purchase from these sites and download them onto your computer (meaning you would not need internet to read them again).
The setting up something to actually play, is difficult too with bearing in mind that: 1) Meeting in person is currently impossible, 2) Internet is poor quality.
If official rpg websites (such as RollD20) do not work for you, then perhaps the better alternative is through a venue such as “Discord”. This is said, not knowing how poor your internet connection is.
But, if you’re not familiar with it: with Discord you can message each other there, outside of sessions – such as to set up characters, before a session – too for people to read and respond to when they can. Practically, I would advise having that all sorted: before you actually try to run proper.
You can also run audio calls, for the sake of the actual session.