Unable to connect iPhones after MAC address filtering to TP link router TL-WR840N

It is common for an ADSL/DSL modem to learn the MAC address of the connected device and only work for that MAC address. Usually, this is done when the modem powers up. In other cases the MAC address is registered an upstream server run by the ISP.

In some cases, the memorized MAC address expires after some time. I would expect this occurs when the DHCP lease expires. The MAC address may be memorized on the ISP's DHCP server, rather than the modem. (Your modem will be registered with them as an authorized device.) In this case, the first device to ask for an address after the lease expires will be allowed to register. I expect this is what you encountered. You could verify this by disconnecting your computer until the lease expires and connecting a different device.

I believe that some ISPs use a DHCP server which tracks leases based on the ADSL/DSL modem's identification. The lease then tracks the MAC address, assigned IP address, and lease expriration time for each modem. Normally, the DHCP server would track MAC addresses, the assigned IP address, and expiration time. In both cases additional information provided in the DHCP request such as the device's name may also be tracked.

Many Home/Office routers can be configured to clone the MAC address of a connected computer. This allows them to take over an existing DHCP lease. Resetting the DIR-120 to factory settings would have cleared any cloned MAC address preventing you from regaining a lease tied to a cloned MAC address.

You may be able to switch MAC addresses by triggering a DHCP release before disconnecting the computer. This should end the existing lease, allowing you to obtain a new lease using a new MAC address. Obtaining a lease with a new MAC address will likely get you a new IP address. DHCP renewals usually don't change the IP address.