using a Max-BR1-Mini (with just cellular) connected to a Pump Station PLC.
The HQ site has 2 internet connections for redundancy… MAIN- Comcast - SECONDARY DSL
if the PLC sends out an alarm to the HQ and the main HQ connection is down, how does the BR1 know to send it to the secondary connection
If you are using PepVPN to link the two sites (with a Peplink balance at the HQ site) then the BR1 mini will know the main WAN is down and use the 2nd.
How would this work with 100+ end points…
In exactly the same way.
You would be limited to 4000 end points with a single balance at the HQ office (a limit on the Balance 2500 which supports that many remote peers), i suspect the first limiting factor would be bandwidth availability on the DSL and that would restrict the number of supported endpoints in a failover situation.
But you could add more DSLs or even cellular at the HQ to give you more bandwidth and with a full balance appliance at the HQ - with speedfusion supported there, you could aggregate all bandwidth across all links for the traffic to and from the remote PLCs and benefit from seamless packet level failover when any one of those WAN link fails.