First, I’m assuming that your Reminders are stored in iCloud rather than locally. If they’re local, I have no advice and you should disregard the rest of this post.
I have had entries in Notes (that were stored in iCloud) fail to update on at least two occasions. The first was while setting up a new Mac that came with Sequoia; the second was yesterday, when I logged in as Admin for the first time in a while. On the first occasion, I simply let the Mac ponder the error of its ways for quite a while, and the entries in Notes began to appear, slowly. Yesterday, I found something amiss, logged out of the Admin account, logged back into the Admin account, and Notes magically synchronized.
The point of all this is that I would encourage you to make sure all your settings are correct before trying to reload your Reminders from a backup. (I wouldn’t know how to do that or even if it could be done.) Make sure that synchronization with iCloud is enabled, that you are logged into your Apple Account, and anything else you can think of. (I don’t know why it would matter in this case, but also confirm that Optimize this Mac is set the way you want. Apple helpfully changes my settings there occasionally.) Then, based on my experience yesterday, log out (of the account on the Mac, not your Apple Account) and log back in. Good luck.