You can use a static bicycle to help with flexion exercises. Here is what I have just started to use
Yes it's a child's bike, set up in my cellar on a static trainer (called a 'turbotrainer' in UK for no reason I can understand). The trainer can provide resistance, obv, but I have set it up with none at all.
The reason for using a child's bike is that the saddle can be set pretty low. At the early stage of recovery an adult's bike would be better, with the saddle set very high.
To start with you can set the pedals horizontal and just rock backwards and forwards if you can't make a full rotation.
Then as things progress you do a full rotation, nice and gentle, then keep going until you can easily turn the pedals at maybe 50-60rpm for 20sec or so. Remember, no resistance, this is a stretching exercise not a training one.
You can see below that using a child's bike that there is a huge amount of scope to lower the saddle more and more to get the level of flexion you require. I expect to lower the saddle by tiny amounts, just a couple of mm at a time, maybe once a week or less often. It doesn't really matter; as long as the exercise is done several times a day (it takes 30sec or so each time) then progress will come.