The signatures are locally created in Outlook 2021 on each device used for outgoing email. As far as I'm concerned you cannot have the signature set as an embed functionality directly in the mail server. Still, it is worth asking your hosting provider (Microsoft or whoever hosts your IMAP mail server). I spoke to mine, hostcolor.com and ask them if is it possible to do so. I was suggested to use local signatures for the email account on each computer.
Important Suggestion!
A possible solution for you is to switch back to POP3 and use the option "Leave a copy on the server". This will keep your email messages on the server for a certain period of time - 1 week or 1 month for example - even if they are downloaded on one of your computers. The good thing about POP3 is that in case of a failure of the mail server and loss of that, you'll still have the emails stored locally in your Outlook. If you use POP3 email on 2 computers with the setting "Leave a copy on the server", then you will have a local backup of your email data on 2 different devices. So, in case of a data loss on the server, you will not lose your email correspondence. With the IMAP, in an event of data loss on the server, you'd lose your emails (especially if you reconfigure your Outlook with a new empty IMAP server). So, it is important to consult with the entity that provides the email hosting service.