Okay, weird fix but it resolved my issue. Went to the Lightning App Builder for the problem page. On the related list component, set the Related List Type from Enhanced List to Basic List and saved. Then went from Basic to Enhanced, saved again, then reactivated the page. The New buttons are now appearing again.
Unfortunately this much needed button is currently not available. For now you have to create child CMTs from their corresponding pages. e.g. go to Custom Metadata Types, "Manage Records" for the Child CMT, click "New", then manually fill in the parent lookup.
The alternative is to custom build your own UI. You will have to rely on the ...
You need to update the page layout. Follow the below steps:-
setup | Object Manager | select the object( In your case, it will be opportunity) | Page Layout | Open your page layout | Go to the related list | click on the wrench icon | select the field you want to display | save
URL hacking does not work with Lightning.
The recommended workaround is to use Actions.