Simulating IN clause for NoSQL database

So I want to simulate a SELECT * FROM TABLE WHERE ID IN {LIST_OF_IDS} query, I know Im using a NoSQL database (dynamoDB) but I have an index on the field for which I want to execute this query. So mainly I want to get all the posts from a user where the userID (index column) is the given list.

I am using Amplify as my managed backend and using graphql as my API. I know there is no IN keyword to use for graphql (at least nog APPSYNC). I have tried creating a custom resolver from a template provided by AWS like the following

import { util } from '@aws-appsync/utils';

/**
 * Gets items from the DynamoDB tables in batches with the provided `id` keys
 * @param {import('@aws-appsync/utils').Context} ctx the context
 * @returns {import('@aws-appsync/utils').DynamoDBBatchGetItemRequest} the request
 */
export function request(ctx) {
    return {
        operation: 'BatchGetItem',
        tables: {
            Posts: {
                keys: ctx.args.ids.map((id) => util.dynamodb.toMapValues({ id })),
                consistentRead: true,
            },
        },
    };
}

/**
 * Returns the BatchGetItem table items
 * @param {import('@aws-appsync/utils').Context} ctx the context
 * @returns {[*]} the items
 */
export function response(ctx) {
    if (ctx.error) {
        util.error(ctx.error.message, ctx.error.type);
    }
    return ctx.result.data.Posts;
}

But this doesnt seem to work. I would also like to avoid to run a query for each user in the list to retrieve the posts. All help is greatly appreciated, also if you have another architectural solution/proposal to solve this issue I am all ears!