I got this problem fixed on the 450 in a rather unorthodox way. Same iOS weak signal and turned out to be a problem with their Tether app, with needing a firmware update, and also a poor web interface. Got it solved for 450 by doing:

The order in which things are done matters... and the problem is bad for Apple users because Apple devices share wifi connection information across devices. Phone reported weak connection, the other devices just refused to connect or stay connected. I had this problem on a 450 v2. My PCs had no problems and it connected via AES, it was all Apple devices which stopped working (Apple TV, iPads, iPhones) because all Apple devices suddenly thought both my router and my extender's connections were TKIP (the router is actually forced AES only and my Asus router refuses TKIP connections). So, all networks killed for all Apple devices meanwhile PC's were perfectly fine on either network lol. I had been trying to do this on my iPhone with the "Tether" app (my first mistake because the app is what seems to set my extender into a mixed mode: WPA2 AES + TKIP and causes the problems on all Apple devices). If I use it even once to change a setting it goes back into this mode and all Apple devices stop working at once, or slow to an absolute unusable crawl. So first thing, delete the app and NEVER reinstall it, it is outdated, problematic garbage imo. Also, I had to forget ALL router and extender-related wifi settings whatsoever on ALL Apple devices and unplug the extender, then reboot all devices (modem, router, extender) and get back on my Asus Router's WPA2 AES-only network as proper AES and not TKIP. So I forgot all router and extender connections on all Apple devices. 

I then did a factory reset of the extender, then hooked the extender up to my (pc) laptop via an Ethernet cable. I downloaded the RIGHT firmware for my V2 (the version is marked on a sticker on the back), which was the latest V2 firmware so as not to mess up the extender. I was careful not to power off or disconnect either device during the firmware update so as not to mess up the extender. When it was done I used the WEB interface only! At this point on V2 I still didn’t get any options for AES in the web interface, UNTIL I changed the MODE away from being an extender. I then got the AES settings... so I set it up this way via web interface, then switched the mode back to extender mode. Connecting devices at this point had the same problem, so don't bother (and if you did "forget this network" on any device you did that on and ALL Apple devices that "helpfully" SHARE these wirelessly across the network). Instead, THROUGH the web interface I then REBOOTED the extender and freshly reconnected and re-inputted passwords on Apple devices. It was only AFTER this extender reboot via web interface that connecting with an Apple device would no longer report weak signal and all devices could connect again via AES and play nice along with my PCs without problems. If I use the stupid "Tether" app EVEN ONCE after this to change a setting, it will again break the network and cause weak security warnings on iOS devices or just kill their connectivity, so don't do that. Leave the app in the bin where it belongs.

I suspect the problem may be similar for other users. I tried absolutely everything, it wasn't until I got it in this magical order that everything went back to normal and started working again!  The company needs to fix their app or remove it from the store and fix the firmware to allow disabling of TKIP (frankly it is bad it doesn't allow this), or change the packaging and help docs to stop suggesting that users set it up using the app...plus prod users to forget the network if they have problems and especially to force an extender reboot via web interface after settings change, for cripes sakes! Lol 😂 All works fine for me now, no weak security, no Asus router refusing the connections.

@tomd Where can i down load the latest firmware?

@TP-Link 

please provide a fix for RE200 V1.

@Solla-topee 

I have an RE 450 v2 with the latest firmware. I keep getting weak security on my iPad iOS 14.6 and  iPhone X iOS 14.7.1. What else needs to be done to resolve this issue?

@Solla-topee I've RE200 V2 and upgraded with beta provided and issue isn't resolved.

I still see weak security for my devices.

@TP-Link  Do you guys have any fix for RE200 v2 version, there are no releases recently.