I was outside without phone just now, so couldn't update this.
Anyway, I went to Samsung after work & I showed them the photo showing the error message i posted above (I never root so I have nothing to hide). The lady went inside & talked to the manager & technician, then she came out & approved the 1 to 1 exchange. I didn't even have to proof anything because they never mention anything about rooting.
I even asked her what does the code mean, why it says "custom" & whether is my firmware official, she said it could be hardware failure.
But since I didn't bring my receipt with me just now, they can only initiate the exchange once I bring the receipt tomorrow. Well, finger's crossed!
For disclaimer, before someone put words in my mouth, I never accuse Red White Mobile of selling rooted or telco set. The root is only released a few days ago, so it wasn't out yet when I bought. Samsung also never mention rooting. As for whether telco set or not, it was just my assumption that I saw M1 apps, but after the factory reset they were gone. So I never explicitly say RWM sell telco set.