SOAP M Object Opportunities Not Accessible via REST API - Need Migration Guidance

[dshchyhlo]

We have opportunities created via SOAP `syncMObjects` API that are not accessible through the REST API. We need to understand the relationship between SOAP M Objects and REST objects, and how to properly migrate or access this data.
I see similar question without the answer https://nation.marketo.com/t5/product-discussions/issue-s-with-switching-from-soap-api-to-rest-api-f....

 **SOAP Opportunity Creation (Working):**
   {
     "opportunity": {
       "type": "Opportunity",
       "id": 18577,
       "externalKey": {
         "attrName": "opportunities.id",
         "attrValue": "8bb9da36-ab39-11f0-80e8-cefbdec3b66f"
       },
       "attribList": {
         "attrib": [
           {"name": "Name", "value": "NAme"},
           {"name": "Amount", "value": "99"},
           {"name": "Stage", "value": "In Progress"}
         ]
       }
     },
     "action": "UPSERT"
   }

- `getMObjects` with ID 18577 successfully returns opportunity data

but all REST API attempts are failed.

Can anybody please help me with that?

[SanfordWhiteman]

It would be better if you posted the actual SOAP payload, not an intermediate JSON representation.