As I am aware Russian iPhone wi-fi specs doesn't differ from other countries, but for cellular yes they may have support for different bands depending on the region.
https://support.apple.com/en-ae/guide/deployment-reference-ios/apd1c22e481c/web
I would say follow @marce1000 advise and grab a debug client output from the WLC and run it against the debug analyzer. it should show you the deauth reason. From the following table you can know why WLC de-authenticated the client.
https://community.cisco.com/t5/wireless-mobility-documents/802-11-association-status-802-11-deauth-reason-codes/ta-p/3148055
Also I would suggest you to check whether any manual overrides are done for AP TPC levels, channel width and check what's the lowest data rate set to mandatory.
As a rule of thumb, I follow the below when designing WiFi for Apple devices. This may change as per your site RF conditions, but for most this should work. So incase if you are redesigning you can consider the below.
1. Make use of 802.11k,v,r, for 802.11r I would strongly recommend for 802.1x deployments, for PSK you can test and deploy it yourself (but really not required)
2. Make use of the 5GHz, have the band select turned on, and you may also consider shutting down certain 2.4 radios in overlapping areas.
3. Have a well planned channel plan. Monitor the RF environment and remove the sticky neighbor channels from yours.
4. Disable all lower data rates. I have had good success with 18 mandatory, but your environment can be different. So take your survey findings to decide.
5. Try not to exceed 25 clients per AP.
6. Make sure the devices are upgraded, this will make sure latest wi-fi drivers are installed in devices.
7. Try not to exceed 21dbm for 5 GHz.
8. Use narrow channel width's.
