My Wife's Sprint Galaxy S8 did the most recent update back on March 25th and has had the LTE completely drop, sometimes will get a 3G signal, but mostly will just get a cell signal. We have been to a Sprint Tech store 3 times over the past 2 weeks now. Finally today, we were told that it is a known S8 issue with the update. The techs have been great, but SPRINT itself is a pain to deal with.
Prior to this they replaced the SIM Card 3 times, did soft and hard resets, and sent it to SPRINT for repair. We were told we have to wait until we get a software fix from SPRINT. Still trying to find out if we can get a credit of some sort for the data plan we are paying for that cannot be used.