I finally figured out how to change LTE band with S8. My earlier complaint about the gaming lag is not due to soc overheating like I initially claimed. It's due to this LTE band selection. It only affect Maxis user since they are the only one with two band (not sure if digi,celcom etc still stuck with Band 7 @2600MHz).
Maxis operate on two 4G band aka band 3 (1800mhz) or band 7 (2600mhz). Normally it wouldn't cause any issue especially when you're outdoor. But when indoors, it really matters. It's alike the situation with dual band router. Lower frequency number gives better wall penetrating while higher number give better speed. The S8 seem to prefer Band7 for whatever reason.
First, dial *#2263#. Then select your sim (stack) for your data. Then go to band selection and make sure automatic is selected.
Next press the 3 dots and back to go back to previous menu. Now goto band disable>LTE. Press reset. Then you will see an asterick for each band. Untick for everything EXCEPT BAND 3. Once you're done, you can end it. You'll most probably noticed your indoor 4G signal improved somewhat.
You can check before and after result by using *#0011#. It would show you the band it is using and the signal strength. Lower number is better for signal strength.
Also take note that you don't need to do this if you're getting 4G+. 4G+ will use more than one band, therefore it will be using both bands.
After select *#2263#, there appear sim 1 & sim 2.
Whats the difference between this two? Scared i salah tekan then my sim something wrong later