I doubt you will actually burn anything if you move from 20mW to 30mW, it is very low power both of them.
And it is probably the same hardware anyway, the output power is usually controlled by hardware locks on newer devices so it is impossible by law to change tx power or software controlled on older devices.
18dB with default settings
What does iw reg get say at default settings without a specified country set?
https://www.rapidtables.com/convert/power/mW_to_dBm.html
But 30mW from the US driver setting is 14dBm plus 5dBm gain for the antenna according to datasheet and that is 19dBm if I have understand this right?
Which is what is theoretically max allowed in the US?
And you have 18dBm with some decimals by tolerance in the hardware.
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global
country 00: DFS-UNSET
(2402 - 2472 @ 40), (N/A, 20), (N/A)
(2457 - 2482 @ 20), (N/A, 20), (N/A), AUTO-BW, PASSIVE-SCAN
(2474 - 2494 @ 20), (N/A, 20), (N/A), NO-OFDM, PASSIVE-SCAN
(5170 - 5250 @ 80), (N/A, 20), (N/A), AUTO-BW
(5250 - 5330 @ 80), (N/A, 20), (0 ms), DFS, AUTO-BW, PASSIVE-SCAN
(5490 - 5730 @ 160), (N/A, 20), (0 ms), DFS, PASSIVE-SCAN
(5735 - 5835 @ 80), (N/A, 20), (N/A), PASSIVE-SCAN
(57240 - 63720 @ 2160), (N/A, 0), (N/A)
A few things....
"18" or whatever power setting number is in dBm, which is dB above 1 milliwatt. So, 18dBm is about 63mw, 20dbm os 100mw, and 30dbm is 1000mw or a full watt!
As to what the chip can do, look up, or look inside, and find the specs of that wifi chip.
Something to think about, vs "more power = better" is that every station that hears you will occasionally answer back to you, eating up airtime. So, you might benefit from LESS power, as long as you have enough to make a reasonable connection to your farthest device.
Another thing, you might think about going to 20mhz vs 40mhz, yes, not faster, but if you are at all crowded on 2.4ghz, you will be straddling 3 channels instead of a bit more than 1, with more stations interfering w you/you interfering w them than if you were on 20mhz. If you are out in the country with few wifi neighbors, go for 40mhz and crank the power, but in a city with dozens of users close, youll probably get MORE thruput with narrower bandwith and lower power.
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I'll add two other things to consider:
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boosting tx power won't necessarily improve overall wifi coverage if the radio rx path isn't sensitive enough. Think of the fact that your sta mode devices (phone, laptop, etc.) won't necessarily be all that powerful on the tx side... so your AP is screaming and the phone can hear it, but your AP may not be able to hear the whisper that is coming from your phone.
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to increase coverage, the best option is to add APs to provide coverage where the existing AP cannot reach. Then, the power on each can actually be lower and the client devices will roam from AP to AP as needed.
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Voyager 1 and 2 only pings with 20W transmission power. We on earth has only build larger and larger antenna arrays through the decenniums to be able to keep hearing the two pings from outside the solar system.
But we can’t send anything to the spacecraft since their receivers can’t hear the noise that arrives at them.
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You can check the regulatory database for the current power limit. It is recommended to set the country code the same as your current location and not to use channels and power level not permitted in your country. In some cases the latter could be considered illegal/criminal. If the database in your router is outdated, you may try with some more recent firmware. Also, there is a radio firmware limit in place - it may contain an outdated regulatory information. Inside the radio firmware there are some power levels for each data rate/modulation. These values are likely some sort of fail-safe, but if you increase the target power, there is a high chance to transmit just junk when using the higher data rates, causing interference without any improvement (even under the regulatory power levels), so I wouldn't recommend wasting your time with any patched firmware. You may try with the latest stock firmware - it could update it. Or it could be non-updatable, embedded in the chip OTP memory... 
If you are using only one TX antenna - 18dBm is normal. 2x18dBm will do 21dBm with two antennas. If you are somewhere in Europe for example, the limit is likely 20dBm.
I would recommend you to open the router and to clean the dust inside (if used) and to check the solder joints of the antennas. This could greatly improve the coverage and the performance. You may also try placing it better, avoiding the signall to pass multiple walls, also changing the channel.
Possibly your device is some EOL replacement, just having the same case and model number, but not exactly the performance of the original WR841n and there was possibly not the same effort designing it. I would't not buy this one, brand new right now.
The OpenWrt Wiki usually has some detailed hardware pages, so you can check the datasheets and in the forum, before buying a device. This is also a good way to find the necessary information to select a decent second-hand router, possibly cheaper than this new outdated copy.
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