Thank you very much for your response. The problem is indeed with the IOS clients since Android clients are able to communicate with the server after the cert renewal without any problem.
Searching on this forum and others, it seems this problem was wide spread with IOS based clients and so I thought may it they didn’t require the fullchain.pem and instead there is another workaround I am not aware of.
Thanks once again. I will keep searching why the cert is cached in the IOS clients and if there is any working solution.