Something is obviously not working as designed and you could spend a long time troubleshooting and tracking down the problem.
Time Machine backup, Format and Install Mojave and move back your apps/doc/pics/movies with TM
I upgraded my ssd with an adapter and the Evo970 Nvme on my MBP 13' early 2015.
I clean installed Mojave from USB stick and since there is serious trouble with bluetooth and wifi.
WiFi: there are two version of the problem.
Case 1: I can connect to the network (WPA 2 Personal) as shown under network. But there is on internet connection.
Case 2: it tries ages to connect and show later the “move closer to router…” message. I’m sitting next to the router - so pointless.
Simultaneously connected devices on the same network show no problem with internet connection. so router problem can be ruled out
Bluetooth: mostly devices can't be found. after deleting bluetooth in library (com.apple.Bluetooth.plist), resetting bluetooth adapter or restarting it may work for couple minutes. But still most devices not detected.
Mojave 10.14.5 was installed before the upgrade and everything worked flawlessly.
Of course I tried google and the solutions pointed there (among others https://macpaw.com/how-to/fix-macos-mojave-problems#part4). But as the say “two heads are better than one”
Hope dearly anyone of you has an idea who helps me. A laptop without bt or wifi is just ridiculous to use.
many thanks in advance!