Diagnostic: in Seiza, your feet should sit open — toes in, heels away (east–west). If they sit straight (north–south), the foot + ankle (especially the outer ankle) is locked up and you cannot sit properly.
Setup for the opening stretch: foot, ankle and shin sealed to the ground — stay active through that area the entire time so the stretch targets the locked outer ankle.
The stretch that opens the foot and ankle: hips back, head forward, and rotate everything toward the front knee. Hips back + head forward lengthens and decompresses the spine, and opening the hip area opens the foot and ankle.