If you have since updated your iPhone you may see the Deco Wi-Fi shown as having weak security. This is due to the Deco supporting WPA/WPA2 (TKIP) by default for compatibility purposes. The iPhone now recognizes WPA as a weak security type. This may also be the same with other security suites with the similar notification.
If you feel you would like to switch your wireless security to either WPA2-PSK (AES) or WPA2-PSK (AES)+WPA-PSK (TKIP) you can download and install the following beta firmware. Future firmware will also have that ability in the future.
User method 2 of this FAQ that shows how you would upload the firmware: https://www.tp-link.com/us/support/faq/1599/
*Note* If you have the Deco M9 Plus v2, and you currently have firmware 1.2.x ("x" being any number), then you need to download this firmware, extract it, and use the utility which is linked in the .pdf to update it (link)
Deco M5_V1/V2/V3: https://static.tp-link.com/2020/202009/20200921/Deco_M5_1.4.9.zip