Was Oreo on the phone when you got it? If it wasn't and you updated from Nougat to Oreo did you clear the cache partition in recovery mode? This was what Samsung and Verizon asked me first when I called and complain about all the problems my phone was having after a major update. This is supposed to help with most problem battery -settings and other issues. The problem you are having with the message+ app maybe a separate issue however because a lot of people are having problems with this app no matter what operating system you have
Turn the phone off then press and hold volume up-power-home
Then when the phone goes into recovery mode use the volume down button to highlight
clear cache partition then press power then use volume down to say yes press power
again. Then just reboot the phone. This not change any of the settings on the just clears out
a bunch of the old files that maybe conflicting with the new files.
This is for you jrh4054