Check to see what fish, bugs and critters are available each month / which ones won’t be available next month and try to catch them. Otherwise you’ll have to wait months for them to come back into season.
I also recommend modding your 3ds if you haven’t already. It’s super easy to do and lets you not only make back up saves of your games, but some very clever people at Pretendo are working towards restoring the online servers meaning the Dream Suit and Club Tortimer will hopefully be back up and running one day.