I would like you to attach a troubleshoot file.
Please, replicate the issue first. The best way to do it would be to restart the router and simply connect those devices, so that DHCP exchanges would appear in the logs.
Then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there.
Another helpful thing would be a packet capture file, containing the same DHCP exchanges. For this you would need to download TCPdump from Services -> Package manager, then, as previously, go to System -> Administration -> Troubleshoot section and enable TCP dump for WLAN interface, used by iOS devices. Download the capture file after connecting some iOS device to the router.
Attach the files by editing your question.