That means you want to connect the AP directly to the ISP without a Firewall/router doing the PPPoE? This is not a supported use-case.
You can (which has to be enabled by support and I never tried it) do a client-side-PPPoE-connection, but I don't think that this is what you want:
https://documentation.meraki.com/General_Administration/Other_Topics/Support_for_PPPoE_on_Cisco_Mera...
I think you have to place a dedicated device in front of the AP to build the PPPoE connection.
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