When you say servers do you mean your device is accepting multiple customer connection requests?
If so I would simply recommend you install a spare device also at the tower (something like a mAP/mAP lite) that should always have a test PPPoE connection established.
If you used the mAP you could connect both ports and have it do one that is site local (using a static user/pass stored on the MikroTik acting as a PPPoE server) and one that is an account stored on whatever system you are using to manage your customers, so you can see if the PPPoE server is down or if your RADIUS server is the problem.
You would then put a script on this mAP to have it cycle the connections every 5 minutes.
On the PPPoE Server MikroTik you could look for these connections via SNMP (interfaces) or write another script to check if they are available. If one or more is offline then raise an alert by sending an email / sms or posting to Telegram / Slack etc - all of this can be done with a script.
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