Same kind of problem on Nexus 5 OTA-12. Alarms and buttons suddenly got grayed out. Restart did not change this. It was not possible to swipe and access the delete checkbox. Solved this by creating a new alarm. Then I could swipe this new alarm, select all alarms and delete all alarms.
Never had any problems with the alarms before.

@ut yesterday again had the same problem: no Wi-Fi button in System Settings -> Wi-Fi. It appears there is also a Wi-Fi button in System Settings -> Battery but toggling this did not appear to change anything (but maybe i am inpatient). Discovered another Wi-Fi button in the top (notifications) bar -> Network. Switching this to On gave me Wi-Fi again and the Wi-Fi button magically reappeared in System Settings -> Wi-Fi.

@MarkG_108 had the same problem mentioned in the first post: System Settings -> Wi-Fi only shows "Previous networks", no Wi-Fi button. Also dekko2 crashed when I tap on a mail subject in the list to open. Dekko2 had been running fine before, although sometimes I had to restart the phone.

Never saw this before and restart did not solve the problem.

I had three apps installed: dekko2, sudoku js (played often), fishy shooter (never tried this). It appeared sudoku js suddenly had disappeared.

I uninstalled dekko2 and fishy shooter, restarted and the problem was gone: System Settings -> Wi-Fi showed the Wi-Fi button again ... magic ...

Running Ubuntu 16.04 (OTA-12) on OPO.

Does anyone know where are the DNS server settings (cellular) and how to change/set them to servers of own preference? Thanks.

Update 15 Nov 2019

To check the DNS servers used you can use nmcli (using adb shell):

1. Get devices

nmcli d

Result on my phone:

DEVICE  TYPE      STATE        CONNECTION                
ril_0   gsm       connected    /204163554125122/context2 
wlan0   wifi      unavailable  --                        
sit0    iptunnel  unmanaged    --                        
lo      loopback  unmanaged    --                        

2. Get device info:

nmcli device show ril_0

Result on my phone:

GENERAL.DEVICE:                         ril_0
GENERAL.TYPE:                           gsm
GENERAL.HWADDR:                         (unknown)
GENERAL.MTU:                            0
GENERAL.STATE:                          100 (connected)
GENERAL.CONNECTION:                     /204163554125122/context2
GENERAL.CON-PATH:                       /org/freedesktop/NetworkManager/ActiveConnection/1
IP4.ADDRESS[1]:                         178.227.211.124/24
IP4.GATEWAY:                            178.227.211.125
IP4.DNS[1]:                             84.241.227.81 <=== T-Mobile DNS server
IP4.DNS[2]:                             84.241.227.82 <=== T-Mobile DNS server