I think you are confusing WAN and DIA (internet), WAN is generally used for connections between sites, either directly cabled or using provider technology like MPLS, VPLS or SD-WAN. If I've understood correctly, then the router at site A must be a firewall of some sort. You could use bridged mode and then if the MPLS devices are set up correctly, the sites will talk and you will have a degree of traffic visibility. Otherwise put them in routed mode and configure the WAN ports with your settings and pick a subnet for the LAN at each site.
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