You fight the AT-ATs on two levels - Hoth and Rhen Var Harbor
On Hoth: as the rebels you need to harass the flanks of the AT-ATs so they don't have such a strong escort of soldiers...this will cause the empire to start spawning heavily around you and will take the pressure off the outpost and bunker...essentially keep meleeing in the valley, cutting off the assaults on the two bunkers (bunker and outpost) and you will eventually attrition the empire down to nothing
As the empire, you should accept that you're going to lose the ATATs a couple of times and not obsess over guarding them. Steal a tauntaun, if you can, and go DIRECTLY to Echo Base. Take it at all costs...this will stop the rebels from having speeders to bring down the ATATs (the speeders can take down ATATs once every minute or so...the gun emplacements take a LONG time to bring them down...get rid of the speeders and the ATATs are fine). You can then take the control and bunker from behind (note: WAIT until you've got some friendly stormtroopers spawning at the echo base CP before going further as the rebels will run in on tauntauns to retake echo base). Alternative to a tauntaun for taking echo base is an AT-ST...along the way, take down the "dish" cannons...this will reduce the pressure on your armor units.
You'll find many of the truths we cling to depend greatly on our point of view -Obi-Wan Kenobi