Any furbaby experts on Glow?🐶

Hey girlies,
I have two young dogs. The female is a little over a year and the male is just over 4 and a half months. 
My SO is preparing for deployment, and he leaves for a six month tour in December. I really want the pups to be as well behaved as I can get them whilst he's away, because things will be stressful enough in our household during that time.
The main problem I'm having is their recall. They go on 2/3 walks per day and they get walked to the meadows near our house on their leads and then let off for about 30-45 minutes on each walk depending on how tired they're feeling. 
The male dogs recall is average, he'll come back and won't venture too far because he's a bit of a scaredy pup, but the female has a horrendous recall. We adopted her from a shelter at 12 weeks so we've had her for about 10 months. 
She just doesn't come back when called, and she's quite a big dog so I fear an extending lead would either snap or be pulled out of my hand. 
I've tried treats and running in the oppposite direction when other dogs or people come, especially with children as I'm sure if she got close enough she'd knock them over! She's a fantastic dog and we love her so much, and if her recall was better things would be perfect. 
Everything we've tried just doesn't work, including a dog whistle and clicker. 
Have any of you got any other advice? I'm so anxious on walks even when my SO comes with me, and I'm dreading when he leaves. 
Thank you in advance!