How do you protect your cob cannons in Survival Endless?

For this particular issue the Split Pea (which shoots two peas backwards) could help. Position it in front of the Cob Cannon so it can cover rear attacks. Attack

The PvZ fandom wiki contains a lot of useful strategies for Survival: Endless btw, give it a read: https://plantsvszombies.fandom.com/wiki/Survival:_Endless/Strategies

Also check the 'tips' section: https://plantsvszombies.fandom.com/wiki/Survival:_Endless?so=search#Tips