Don here from Shopify
As you have seen for yourself, the NMI gateway is no longer supported for use with new Shopify stores.
If you are having any difficulty with finding a gateway that can accommodate your line of business you might want to think about using an alternative gateway such as Emerchant Brokers. You can reach them via this signup link.
They should be able to help set you up with a gateway account that will allow your business type okay. :)
All the best!