WhatsApp Messages Missing after Reinstall
Usually, there are two ways to backup WhatsApp chats to iCloud.

Within the WhatsApp settings, go to Chats and then Chat-Backup. There you have several options, e.g. automatically backup your Chats or exclude videos.
In the settings App, head to Apple-ID, iCloud and then you will find iCloud-Backup. There you can select which apps and documents you want to backup to iCloud, i.e. you can backup all phone data and apps.

If you haven't select/used either one of the above backup options, then you are out of luck. They should both display the date and time when the last backup has been completed, so you should be able to check both.
In your case I assume you have not selected the chat backup method in WhatsApp, but the second one, backing up your complete iPhone. Yet, if the last successful backup was completed one month ago, there is no way to retrieve newer Chats.

As a general rule: If you are not 100% that your data is in a backup, never modify important apps and documents. Especially if your work relies on that. It is definitely worth it spending a - comparatively to the harm it may cause - little money on iCloud Backup or a hard-drive for local backups than to risk losing the data forever.

How to backup Time Machine backup to a Cloud?
Offsite Backup Strategy for multiple Macs

I know this refers to Mac (and TM), but in general - deepening on how important your data on your phone is - the same general backup procedures could be applied to your phone.

Note: There is still the option to backup to a computer. If you have done that, you can also restore the data from there.