I am so sorry to hear what you are going through ..
You have a few options:
A- You can try reasoning, negotiate with your ex. Why is he doing this ? Have you tried turning up at his property and talking face to face ?
B – You can file c100 form for residence order also c79 form (enforcement to the court order). With c100 you can be granted The Residence order but if your ex brakes the court order the police won’t be able to help you. You will be stuck again. £215 for an application and be prepared to wait to hear back from them soon during this pandemic.
From my personal experience never pay for lawyer or solicitor as there are free advise available – I spent thousands yet I stil have to go back to court again and wait …
Hence be strong and wise enough to convince your ex to let you have and share your children