Phone batteries generally lose 20% of capacity every ~500 cycles. So lumost may well have a battery with only 1/2 the original capacity or less. [1]
[1] This is for iPhone, but I imagine it's similar for all devices. https://support.apple.com/en-in/HT208387
Look, it might very well be a shitty phone. Just saying that after 4 years I'd cut any phone a break. Especially since phones got impractically thin, batteries have really struggled to work the way they do when they are new after a year or two.
I still miss the hardware keyboard.