What did you mean by “can see the alerts in the prometheus alertmanager url”? if you go to https://<pmm-server>/alertmanager or are you configuring an external alert manager?
when I’m troubleshooting PMM Alerting (not grafana alerts or External Alertmanager) I first hit the https://<pmm-server>/alertmanager url to see if the alert is firing there. if I see nothing in alertmanager grab the expression (click the Alert Rules tab and the down arrow next to the alert rule name) and paste that into the “Explore” feature of Grafana without the threshold criteria to see what is in the victoriametrics database.
If you’re using an external alertmanager integration (an alertmanager running on a separate container/instance that you configure PMM to point to setup at PMM Settings → Alertmanager Integration) then the communications preferences won’t work with that and you’ll need to setup communication channels directly in that alertmanager instance.