I have tried finding a setting but can’t find an option to increase the attachment size from 18 MB.
I am afraid that there is no method could increase the attachment size from 18 MB.
You could refer to this doc: File naming conventions
Restriction type Restriction File size Must not exceed the maximum of 18 MB Attachment file size Must not exceed the maximum of 19 MB
This is a limitation of Azure DevOps, so there is no method to increase it for the time being.
But the requirement makes sense, you could refer to the following suggestion ticket in Our UserVoice Site: File size limits for wiki are restricting
You can also create a new suggestion ticket based on your requirement.