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You can run a full stack behind the MG, or you can run the MG as an independent 5G gateway - up to you based on the architecture of your site. Keep in mind bandwidth varies by carrier, by region, by time of day even.
Is that connection to the Rete Test Plant MPLS or L2? Have you seen this? https://documentation.meraki.com/MX/Networks_and_Routing/Integrating_an_MPLS_Connection_on_the_MX_LAN