In what capacity? It is not entirely clear.
According to the Azov Commander Andriy Biletsky's official Telegram account (Archive:
Roman, together with Azov and other military units, fought against the
occupation of Ukraine. He was with us near Shyrokyne, where he was
wounded. But his weapon as a journalist wasn’t a gun, but the word.
In this interview, Protesevich basically confirms the above (except that he was with Azov):
I'll tell you more: I was reporting in the ATO [Donbass conflict area]
zone [...] I was wounded, but I was still a journalist
Last summer, (when he first became widely known, but before any accusations of being in Azov), his father said in an interview:
When he was in Donbass and fought on the side of the Ukrainian
He's retracted that comment after his son's arrest.
A reporter from the US-funded RFE/RL subsidiary, Current Time TV, asked Protasevich's father in 2021 why he told their reporter, last year, that Roman had fought.
Protasevich was found in the photos of a 2015 military parade that Azov organized in their ranks, as an ordinary trooper:
So his role was from serving in Azov in a PR role to actually fighting and hiding the fact, and anything in between.
EDIT: oh wow, when looking at the transcript of the YT video, with the interview of Roman's father, in response to a comment about providing a translation, I saw that right after the interview with Roman's father, where I'd stopped watching, they actually have another interview(starts at 9:14, https://youtu.be/PCA5vGPQlUM?t=554 and as a reminder, Current Time TV is US-funded RFE/RL subsidiary) with Vladislav Sobolevsky, the Azov chief of Staff for Mariupol where he confirms the veracity of the photos, including the Black Sun magazine cover, but says that Roman was there as a journalist, not a formal Azov fighter and he just liked to pose with guns in uniform because it was cool. The veracity of the second part of his statement is debatable, since one of the photos he confirms the veracity of is this one:
The censored version of which is from a 2015 Radio Liberty interview with a Belorussian Azov fighter, who's identity was concealed at the time: https://www.svaboda.org/a/27255566.html. It could still be that, just like posing with guns, Roman wanted to be seen as a warrior and lied about his role to the Radio Liberty journalist, but there is no question as to the veracity of the photos.
As to the question of translations, I checked and Google translate works well enough to confirm what I'm saying for the written articles and as for the video, the YouTube transcript can be copy pasted into Google translate, that works well enough for the interview with Roman's father, e.g.:
сегодня днем я созвонилась с отцом
романом он не смог выйти к нам эфир
очень много встреч говорит о не супругой
практически не спят уже третьи сутки я
спросил а почему же тогда в киеве моей
коллеги юлия жукова и он говорил что
роман воевал и вот ответ дмитрия
but not for the part with Azov's chief of staff, since that's Ukranian that's been machine transcribed as Russian(almost all the video is Russian, so the Google bot got confused) and is potato quality. I'd welcome any suggestions for English language sources, but from what I've found, the only good English language articles on the matter are from RT, which I don't want to use, because I'd be accused of sourcing Russian propaganda(I deliberately only linked to neutral, non news sources or American funded media for this reason), while Western media sources don't have the domain knowledge and just seem to take whatever the Ukrainians say on the matter at face value.