If there is ORDER BY and query is not following index order it must fetch all possible documents and do sort (last fetched entry might be first in the order).
The following article gives more details Using OFFSET and Keyset in N1QL | The Couchbase Blog
As no ORDER BY expression in query, it difficult provide index advise. You can use Index Advisor
https://index-advisor.couchbase.com/indexadvisor/#1