you can configure if a host is monitored via agent, snmp, agent and snmp or ping only. Maybe the aws instance is configured to monitor only by ping? You can find these settings in WATO > Host properties > Host tags > Agent type.
For security reasons you should secure the connection between your omd host and the aws instance. The traffic between check_mk agent and server is not encrypted, also there is no authentification implemented.
Thank you for the comment. I was configured by default as "Check_MK agent (server)". I did not know that once the host is added, I need to schedule a re-inventory of the host. I did that (first, forgot to) activate changes and I see multiple services now. Thanks.