You cannot use SNMP exporter for this use case, at best all you’d get would be aggregate counters per network interface or VLAN.
To get per-site data, your best bet is having a device in the line of traffic capable of emitting netflow/ipfix/sflow data. A Mikrotik router demarcating your ISP connection is probably the simplest and most cost effective way to achieve this.
Then you need to direct that data to a pipeline capable of collecting the flow records and storing them in a format capable of being visualized in Grafana. This is the most complex part due to the excessively high cardinality of raw flow data. As a starting point, you could look into GoFlow2+Gobi or Elastiflow+Elastisearch.