Can you share some screenshots of Storage Manager, just to confirm how the storage is configured. As an example here's my DS218+ showing its two drives are configured as SHR-1. If you have a similar setup then you can upgrade the storage using disk of the same or greater size. To realise the increased storage capacity you have to replace both drives.
In addition to the above please add detail such as the usage, if it is your only NAS and if you have a full backup of the NAS.
This is important on a 2-bay as expansion or rebuilds are notoriously fickle at the best of times and you have less options than on a bigger units. There are suggestions in all cases to mitigate the risks so your wider environment may help or need more careful upgrade planning.