Can't enable notifications for SMS messages with MIUI 12

I'm using a Xiaomi 9 SE phone, with MIUI 12.0.2 (global). I'm not getting notifications about incoming SMSes! :-( I tried the general phone settings, under Notifications; the slider toggle for SMSes is enabled, but semi-grayed-out - unlike all other notification toggles. I tried the SMS app's own notification settings (through the top-right gearwheel menu) - and while that has a bunch of settings, it doesn't seem to have the basic setting for notifying me about incoming SMS messages. What do I do? Note: I should stress SMS messages do arrive, and the unread ones are marked as such when I open the SMS app.