You can still view up to 3 months of past memories even if you are not online. To view memories offline, do the following:
Step 1: Open the app and click on the menu from top left corner.
Step 2: Click on Settings.
Step 3: Then click on Backup and restore.
Step 4: Offline memories should be turned off. Click on the arrow and choose from the menu. You can choose to keep 1 months, 2 months or 3 months of memories offline.