Updated yesterday to 14.0.1 on XS Max and iPad Pro (2018). As soon as the update was done both devices showed 4G signal strenght dropping from full to only 1. At the same time measured mobile data speed (Speedtest) dropped on both devices to very low making mobile data connection useless. Calls seems to work, though.
Did all the resets, reboots etc no help. Forcing to 3G did not change anything, ie signal is very low on both devices and mobile data connection is so slow that is not usable at all.
My work phone (XR) is not updated yet and its 4G mobile data works just fine...so these problems are definitely due to the update.
My wife’s iPhone 8 stopped charging after updating to iOS 14.0.0. Got replaced under extended warranty, but needed to pay extra for a new device.
As a long time Apple user I can see a trend here and it is not good...not at all...