Contact support and tell them how many you need and they’ll try to accommodate you. There were a lot of people abusing the service and hosting hundreds of domains so now they’re making everybody request them explicitly unfortunately. They’ve also had to suspend their .dedyn.io DDNS service indefinitely because of the abuse.
That’s why we can’t have nice things.
Please read up on DNSSEC because you will be required to turn it on for every domain you host with them.
SIMs are standalone embedded computers (they run Java!) that handle the cellular connections one their own and communicate with the phone over a standard pin-out and protocol.
This way the phones are somewhat insulated from advances in cellular technology and it’s one of the reasons mobile phones have been able to evolve so smoothly from feature phones to smart phones.