It looks like my workaround does not work 100%. During writing the comment I triggered the bug again and again. After it also with a WLAN connection I could not enable notifications. Luckily rebooting my mobile solved the problem. After it I could enable notifications over WLAN.
Also it looks like you do not need to close Telegram, just close the settings menu and enter it again is enough.
It looks like my workaround does not work 100%. During writing the comment I triggered the bug again and again. After it also with a WLAN connection I could not enable notifications. Luckily rebooting my mobile solved the problem. After it I could enable notifications over WLAN.
Also it looks like you do not need to close Telegram, just close the settings menu and enter it again is enough.
My Telegram version is 2.2.30.0