Why instant approval does not exist
A merchant account is a line of credit. The acquirer is liable for your chargebacks if you cannot cover them, so a human underwriter reviews your business before approval: ownership, processing history, website compliance, refund policy, financials and sector risk. That review takes days at minimum, and high risk files take longer because they go to a senior underwriter or a risk committee.
Providers advertising instant approval are doing one of two things: boarding you as a sub-merchant under an aggregate account, which can be switched off just as instantly, or approving you subject to later review, which is how accounts get frozen in month two. Fast onboarding that skips underwriting is not a feature. It is the reason most high risk merchants lose their first account.