although theoretically doable, i cannot see how this could help

in a nutshell, the html output is something like this

....
<tr><td>some label</td><td>some value</td></tr>
...

so just adding a <br> before/after the label / value will still keep it all in their own table column
adding a <br> after the closing </tr> is simply invalid html (and will therefore in most cases be ignored probably anyway)

are you sure your printer can actually print html in the first place ?
have you tried using a plaintext template with that printer ?

> I have this problem in many thermo printers.

I am sure you do.

<rant>
(the majority of) thermal printers are technologically still stuck in the 80's 
from what I can tell
All I (i.e wppizza) can do is output valid, standard html and/or plain-text 
documents (and with the pdf plugin pdf's too - as attachments) that can be 
send to and printed by a/the printer. 
It is completely out of my hands as to what the printer does with this 
when printing.
From what I can tell, most thermo printers simply do not follow *any* standard 
and/or do their own thing as to whatever the company has dreamt up at the 
time the format should be without any consideration at all as to any standards.
Afaik, in many cases one cannnot even select the most basic things like  fonts or fontsizes , 
let alone anything more specific....
I must admit, I find the arrogance/laziness (call it what you will)  
of printer manufacturers on that front quite staggering to be honest .....
(i could rant on about this for quite a while here, but it wont help you of course, 
so i wont ------- for now :))
</rant>

So, that off my chest (sorry) back to the issue at hand:
(You did not address this question) – Did you try setting your template you are using for printing to plaintext ?
is the output better/worse ?
perhaps could you post a screenshot of both outcomes ?

Btw, I’m not trying to be difficult, just trying to find the most appropriate workaround/solution to what really should be someone elses problem (I’m referring to printer manufacturers here, please don’t take this as a dig at you !)
to perhaps come up with something that helps not only in your particular case but might be generally useful for others too coming across this post…..

thanks for the screenshots.
from what i can see, your issue isn’t actually the html/css as the printer does seemingly print bold and table columns etc (referring to the screenshot https://prnt.sc/qito1l) but that it forces left and right margins .

The most appropriate solution I would think is setting the margins on the printer to something smaller / zero.
However it might be the case that the printer does not allow for this to be adjusted.
If that is the case, my idea would be to add the following to the css declarations

a) set the body margins in the css declaration to something negative . i.e

body {
margin-left: -100px;
margin-right: -25px;
}

or perhaps in mm

body {
margin-left: -10mm;
margin-right: -5mm;
}

(you might have to play with the values until it fits)

b) regarding customer labels, add the following

#customer tbody > tr > td:first-child {
display: none;
}

which should hide the labels there

always provided the printer does in fact understand this sort of thing.
let me know if the above makes any difference please