In order to buy expansions, go to the "Store" tab to the right of the "Account" tab. You can buy Silver (roughly half) or Gold (all) expansions, for a duration from 1 month to 9 month at the moment.
If you are still playing with those expansions, I assume you predominantly (or exclusively) play against other players, not against bots. If you start a bot game, you should only be able to get cards from the base game. In a game between human players, the Kingdom is constructed from the whole card set that all involved players own.
The reason you are not getting as many games as before is most likely related to your subscription running out. In the "Matching" tab (leftmost tab), if you click "Advanved Options", you get to the area where you can adjust Min/Max ratings settings as you mentioned. It also contains a field "Waiting for Expansions" right on top of the Min/Max.
This field, if checked, means that you want to wait for a match until at least one involved player owns "a few" expansions (exact details for "a few" are not important). When you had all the expansions, that was automatically fullfilled for you, that means you were a 'desireable player' for all the players without subscription that want to play a game with cards other from the base set, and the automatcher could match you fast with any such player.
Now that you are yourself without subscription, you are not a match for those player anymore, so you get less potential players to match against.