How do I reserve seats on an MH codeshare? MH won't help

I am booked in j on an Oman air ticket from FRA to Mel via KL

Fra to Kl on Oman air and Kl to Mel on mh

I can assign seats on the Oman air legs, not on mh, either on Oman air or mh websites

Called mh but the have told me they cannot assign seats, even as the operating carrier as the ticket is through Oman air. Their helpful suggestion was book at checkin, great as I am flyin as a group of 4 with 2 kids. I asked to escalate to a call centre manager and they disconnected the call😡

Any advice on who to contact to fix this?

Have you called Oman Air?

I have emailed but no answer by phone

I would be surprised if they can allocate on mh metal?

Sometimes they can, it's definitely worth a try.