First, let's flip the Oppo R9 gently to the back to reveal its delicate card slot design. The phone's unique single-slot, dual-slot design means it's capable of accommodating two Nano-SIM cards at the same time, providing users with great convenience. Next, we need to use the pin tool to open the card slot. The process is very simple, with a single pick, the card slot will open smoothly. Then, we put the two nano-SIM cards in the corresponding positions according to the directions indicated on the card slot. This process requires careful attention to the instructions to ensure that each card is placed correctly. 
Then, we reinsert the card slot into the phone and make sure it's securely secured. This is a simple step, but it's an important part of making sure your phone is working properly. Once we have completed the above steps, we will only have to turn on the phone and enter the settings menu of the mobile. In the menu, we need to select the option "Dual SIM with Mobile Network". Here, we can set the network of the two cards in detail, including network mode, data traffic, etc. Once we've done that, we've successfully set up the second card. 
It's worth noting that when we place the second card, we need to make sure that the carriers of the two cards are the same. If the carriers are different, the signal may be weak or not working properly. At the same time, when using dual SIM cards, we also need to pay attention to the use of the primary and secondary cards to avoid affecting the normal use of the mobile phone due to improper use. 