I suffer a serious problem after updating to Oreo from Nougat on Galaxy S7 (flat), the problem is High percentage of Cell Standby drain of Battery when I use Mobile data (4g/3g etc.). It is the first consuming factor (for about 40 to 50% of battery usage, while screen is about 15 to 20%). My phone is awakw for just about 4 to 5 hours.
It doesn't depend to the strength of the beam or antenna, or 4G or 3G. Surprisingly using Wi-Fi for the net, doesn't have any problem with Cell Standby battery usage (just about 2-3% usage).
After surfing the net for few days, I recognized that it is not just my problem, and it is common unfortunately :