Because the new tenant told me he wouldn't financially qualify by himself (only collects a little bit of disability) and the original tenant didn't want the rent to go up, the original tenant sent me an email putting in his 60-day notice to vacate the premises. 

To be on solid footing if there were ever an issue to arise, you really have two options 1) collect rent and then since you are eliminating their use of the unit put them up somewhere 2) provide a credit or reduced rent for the days the unit is taken out of service. 

You may have triggered additional income taxes that you would not have had if you had collected rent monthly. 

What are the best ways to collect rent on time, what apps, websites, etc?

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The 2nd reason I closed the LLC's is because people can still sue you, personally, and they can easily collect their judgements from your LLC's.

Not every city is favorable to Airbnb and you have to do that research upfront before you waste time and money.Beyond that, success in Airbnb isn’t as simple as buying a house, listing it, and collecting cash flow.

@jason chaseWhat your lender is asking for is what we commonly call a "comfort letter".It is a checkbox on their list, and will be scrutinized with the attempt to shift liability to the accounting firm that wrote it should the loan go south and the lender cannot collect from the lendee.The AICPA as well as errors & omissions insurers strongly discourage tax and accounting firms from writing these letters. 

Investing with the idea rhat you will collect rent to help pay for your duplex will not make you much of a profit and will not keep up with inflation.

And don't hesitate to send it to collections.