I was having a similar problem for my site that I imported from blogger and I did not know SQL.
So I installed this plugin search and replace which did the SQL replace part for you. Plugin Link below:
https://www.ads-software.com/extend/plugins/search-and-replace/
In the “Search for:” section put
In the “Replaced with” section put <!–more–>
Click Go and you are done.
Once you install the plugin, you can access the plugin interface directly from the plugins install page of wordpress. It doesnt create a new tab in settings or anything.