Thank you for clarifying your surgical history.
From my perspective, you have two related but independent issues: running and dorsiflexion.
You report you lengthened 6 cm. A good rule of thumb is two months consolidation for every centimeter lengthened for full bone recovery. This means your bones will likely be largely healed in 10 months from now (6 cm x 2 months = 12 months - 2 months for consolidation already underway = 10 months).
Yet, you want to run in 4 months. Perhaps your timeline is too aggressive?
While many internal nails are fully weight bearing, bearing weight and running are not the same. Run too early and you risk shearing off the screws that hold the nail in place. Do that, and you will drive your nail through your ankle or your knee--bad either way.
So, has your surgeon approved running in four months?
As for dorsiflexion, you are only two months into consolidation and your equinas is not bad. However, your flexion will NOT improve on its own. Half-measures are rarely productive. Gadgets cannot replace good, hard, fundamental work.
If you are approved for weight-bearing; then, get on your feet! Doing so promotes bone growth and improves flexibility.
Stand, hold onto something, and concentrate on relaxing until your legs are straight and your heels are on the ground. Alternate pressure by putting all of your weight on one leg for several minutes; then, switch. Repeat this process for at least 30 minutes, at least 3 times per day.
Make sure you get good rest too because rest is a necessary part of recovery.
Best of luck!