In order to unlock the Canon, you need to make it through Wave 20 in the HTL (Hold the line) game mode. The Canon is a very powerful weapon to win later waves in the Open Battlefield game mode.
Anyway, just go to the top of the screen under Modes and select the Hold The Line game mode.

I'm just describing a simple strategy i used to make it past Wave 20 to get that sweet Canon and i'm sure there are other ways to do it, too. Feel free to share them in the comments! :)
So, just try to get as much money as possible to spam simple SMG and Bazooka soldiers.
I ignore Upgrades for my soldiers for now and only focus on getting Extra Reward Upgrades or the Get 200 Coins Reward if necessary. Reinforcement Rewards in general.
Around Wave 10 or 11 i still only use the normal SMG and the Bazooka and didn't upgrade them yet.

So you easily cruised to Wave 18 now and have been busy with putting mines on the Battlefield and fighting those hard attacks on your left flank. From here on til Wave 20 i only use Artillery Strike Reinforcements to help my brave soldiers.
You should have plenty of Money to spam simple SMG (1) and Bazooka (2) in a tidy line.
With always choosing the Extra Reward Upgrade and getting those 200 coins you will have a easy time going through Wave 20.
Just go for the Reinforcements Rewards.
Watch out to not get motion sick if you spam a lot of Bazooka soldiers. =)
I only use the first, simple SMG because the other SMG will cost you no money but 2 simple SMG soldiers will disappear which gets quite costly.

Congratulations, you made it past Wave 20 and unlocked the Canon!
That was the only thing this guide was made for and not how many waves you can endure in the HTL mode. The moment your Base goes to 0% and you loose the battle, the Canon unlocks and you're good to go to use it in the Open Battlefield!

Have fun!