Oh i understand what you mean now.
Step 1 : settings connections wifi, click the 3 dots, go to intelligent wifi. You have to be connected to wifi for the next step, then you have a “switch to mobile data” setting inside the intelligence wifi, turn it off.
Step 2: settings developer options, “mobile data always active”, turn it off.
I forgot that these 2 settings are on by default as i always turn them off day 1, my bad.
First off wqhd+ doesn’t take any more battery than fhd+, all the pixels, the same exact pixels making up your screen are still physically turned on and use the same amount of power.
Only difference could come in from ui rendering, but as all the ui is 2d, again you aren’t using any more gpu processing power so no power savings from there either.
Only in 3d games will there be a slight difference, for games that actually render at 1440p and in 3d.
If you want to feel placebo you could use the phone on lower res you can but you will not see any actual battery difference.