The charger your link goes to says it will charge battery up to 75 amp-hr and maintain battery up to 120 amp-hr, so NOT big enough for battery that should be fitted to Rangie.
Jump leads on,try to open flap with ign off; then try to start engine.Think that`s the way to go. If no luck then access thro boot etc.
P.S. IF petrol not diesel then maybe that charger IS big enough...just. But a larger than standard battery would be a good way to go if replacing it..