Currently Lambda restricts access to ECR to the same region as the Lambda function (see: here) even though it allows an access to an ECR from another account.
To resolve this, use ECR replication to different regions (or even different account and regions) - see more about this here. Please mind that for a cross-account ECR replication an additional policy is required - see: here.
Note: There is however an open issue regarding the resriction here.