Also in case that is what's missing: you need to link your NamiComi account to your MangaDex account afterwards, otherwise we have no way to know what MD account to give the role to.
You can do so on your user page:
https://mangadex.org/user/me
And if you had linked it prior to subscribing, you can click the refresh button next to your connection line, like so, to force it to re-check:
View attachment 2015