In Meta Ads Editor (bulk upload), with ONE ad account selected, open the collapsible Page and Instagram section before uploading. There you choose the Facebook Page and Instagram account the ads will run on, regardless of what is set on the ad set. The list shows the Pages and Instagram accounts in your Business, with the ad account's own at the top. It's optional: if you leave it on Keep from ad set, ads inherit the ad set's Page as usual. Useful for the first ad in a new ad set, or for uploading to a different Page while keeping the same Instagram account.