As of 27/09/2019 none of the guides are updated. But this solution will help you to clarify things.
Update: Great addition from ShadyShall
: Make sure to write down or memorise the Server number. This way you wont waste time joining to same server if you are too late for the flag.
Battlefield V's framework is completely broken, whenever you press button it will kick you out of the game. So you shouldnt check your achievement progress while playing the game.
First of first, Frontlines only avaliable in Grand Operations. To make it easier for you; uncheck everything on advanced search. Then under maps section only choose these 3 maps: Devastation, Arras, Rotterdam. DO NOT CHOOSE Frontlines Mode, if you do specify the mode nothing will show up on Server list.
After you set these 3 maps, only look for Frontlines games. It will be mixed with other game types. If nothing shows up, refresh by pressing
Pick one of the Frontlines game, the game has to be just started otherwise quit and look for another server. So, some people may not know this but only first objective(flag) counts towards the achievement. Not the other 3 objectives you explode further in game.
After you captured the flag, dont check your progress, instead wait till game ends. Theres always another chance to get 2nd flag, that only happens if you dont explode 3 of the objectives and out of lives.
Since 4.4 update achievement progress is fixed. Feel free to quit the game and look for another Frontlines mode. Your achievement progress wont reset.