Sorry , not "boarding card" - TM6 is the "arrival/departure card" - it used to be stapled into your passport by immigration. It has been discarded for several months to make it easier to process people at immigration.
however, they are now introducing an "online" version. the "TDAC"
How do you access this without an internet connection?
It seems that there will be kiosks at airports in THailand.
I don't know what they will do before you check in in you home country. Maybe the airlines will have some facility?
With the old system they used to hand out the form on the plane but there were still lots of people filling in the forms on arrival before proceeding to immigration.
Thailand has very bad at doing anything online, so I would expect all sorts of problems when it starts - people who didn't know, those who can't get online and of course the system itself not working.
I think Thai immigration are hoping that most people will use the e-Visa portal before they fly.-
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If you arrive without one the airport has free wifi that you can access with you phone so you won't need any local account.
I'd also suggest that regardless of this you get a Thai SIM card or e-SIM for you phone before or after you enter Thailand - it's very cheap and convenient