In most cases to put something back under support, you have two options.
1. You can backfill the missing years of support. So if you bought 1 year, then went three years without any support, on Year 5 if you need support, you would buy 4 years (3 of the previous and then one for the current year).
2. You buy a new airwave license for the number of devices you need to match or exceed what you have.
That is pretty standard across the industry, covering servers, networking, software, etc.