First, it doesn't matter if LMS is a gateway or not.
There are more steps you need to do:
first, you need a number plan (numerotation plan) for your invoices. Configuration - Numberplans - add - template: "Nr %N" where %N is replaced with the invoice number starting from 1.
then you need a tariff (let's say 50$ per month for 10Mbit internet) Finances - new tariff - name: "10Mbit Flat Rate", value: 50, upload and download as you wish your customers to have.
after that we need to assign that tariff to a customer so we enter that customer's page: down the page, assignments, check from the list, add
So you have a customer with a monthly subscription. Now you need to execute from the bin folder in your lms the script lms-payments every day. That script takes the list of assignments and generates the invoices for the current day.
Of course, you can set the day of the month when you want the invoice to be issued and so on, but as a basic method this is it.
This subscriptions-stuff is intended to be used for automatic monthly/daily/yearly invoicing. If you wanna just issue one invoice to a customer you can use the link at the right of the customer balance list (down the page - New document - invoice)
Please excuse my English :)
Hope this helps