In this case, it is best to visit the PF office that has your account and take a hard copy form to fill out. Take the copy and original of your passport-sized photo, Aadhaar card, PAN card, cheque copy with your name pre-printed, or your bank passbook with the IFSC code, matching the name on Aadhaar.
Submit a letter informing that you have a PF but no UAN and collect the form, fill it out, and submit it. You can take the signature of the previous employer or a certain bank manager, etc., which the PF office can guide you on.
Hope this helps. Also, do post your feedback after your case resolves so that it can help others in the same situation to know the process.
Warm Regards, Deena Jagasia