As a wise man once said:
Side kicking – This can help you with both coordination and flexibility and is one more benefit to practicing TI drills. Each Long Axis drill (i.e. those involving flutter kick) start and finish with time spent in your Sweet Spot. Any time you’re kicking on your side, you’re a lot more likely to use the 30-degree flexion kick. Kicking on your stomach – with or without kickboard – makes if far more likely that you’ll do the bicycling kick, because gravity encourages it. On your side, because your knees don’t flex in the direction gravity is working, you’re far less likely to “bicycle.”
So, substitute "side kick more" for "kick more."
You can do several things to increase your ankle flexibility so that your body is physically capable of executing the desired movement pattern, i.e., a hip-driven downward flick of the foot, followed immediately by a return of the foot to a toes-pointed (plantar flexed) streamlined configuration directly behind your hips, instead of hooked (dorsiflexed), drag-creating "Aquabrake" feet. See the Shinji Takeuchi YouTube video at 1:00 to 1:13 to get a good idea of what the flick-and-streamline should look like in two beat mode. First, kicking itself helps (see above), with and without fins -- just practice it with purpose and focus, trying to "myelinize" (get used to) something new and better. Static ankle stretching exercises, if you do them well enough, often enough, and long enough (think daily at least, for several weeks at least) will also help with the ability to achieve a relaxed streamline, so that you can "point your toes" (get the bottom of your feet parallel to the surface when they are not doing anything) without having to engage the soleus and gastrocnemius to pull them into trim, which gets crampy quickly. Static stretching won't help so much for the dynamic "flicking" action -- only kicking really helps there. The simplest stretch is to sit on your ankles with the top of your feet flat on the floor and apply gentle pressure down. That will eventually get your ankle flexible enough that you can actually point your foot straight back. Do the same thing with a rolled up towel under your toes and you can develop another inch or two of mobility. There is no magic -- it just takes persistence. Unless you have some anatomical defect, which is almost certainly not the case, you can significantly improve joint mobility in a sport-specific and useful way. Just know what you want to achieve, for what you need to do, and get that degree of flexibility, and no more -- no need to turn yourself into a Cirque du Soleil flexibility act. You do need to be able to streamline your feet without strain to swim your best.