@XYZZ123 

I'm having the same problem and the same issue was also discussed in another thread (link).
But @Solla-topee provided this answer, which in my opinion can't be accepted as it simply doesn't fix the security issue:

Our Range Extenders use Auto secutiy mode by default, which supports WPA2+AES, and TKIP to be compatible with the devices that only support old encryption methods. If the Apple devices detect that the wireless connection a weak encrypted network,  'the weak securiy' would show up there. But in fact, iOS devices would still perfer to the secure encryption method WPA2+AES, instead of TKIP when connecting to the range extenders, and users do not need to worry about the potential security issue.

KR

@hugo11 

yeah, that's not reassuring at all. is there any verification of this claim that we may find elsewhere?

@XYZZ123 

I got this problem fixed on the 450 v2 (the same issue seems to happen on multiple extender models, so I'm posting this in the off chance it is the same for this one). 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. Phone reported weak connection, other Apple devices refused to connect. PCs had no problems and connected via AES, all Apple devices after using tether app thought both my router and my extender's connections were TKIP (the tether app sets an outdated mixed mode WPA2 AES + TKIP, which is the problem. My Asus router forces AES only for security and refuses TKIP). So, all networks were killed by the sane router for all Apple devices due insecure outdated mode. Got it solved for 450 by doing:

The order in which things are done matters. First thing, delete the tether app and NEVER reinstall it, it is outdated, problematic trash. Apple devices share wifi connection information across devices. So, I had to forget ALL router and extender-related wifi settings whatsoever on ALL Apple devices, then unplug the extender, then reboot all devices. 

I then did a factory reset of the extender, 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. Don't bother connecting devices at this point, you'll have the same problem. Instead, though the WEB interface REBOOT 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 no longer reported weak signal and all devices could connect again via AES and play nice. 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 due to smart router refusing the poor security outdated TKIP mode the extender tries to use, 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 posted, 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), ...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! Their own supposed help docs for this skip this vital step and I ended up with the same issue Lol 😂 All works fine for me now, no weak security, no Asus router refusing weak TKIP connections. Good luck!

@Solla-topee 

there is no any firmware update for RE200 v2 since 2018

https://www.tp-link.com/us/support/download/re200/v2/#Firmware