The mobile first index is not live as of today
This is from March of this year
http://searchengineland.com/googles-mobile-first-index-still-months-away-271851
"Don’t freak out. That’s what Google’s Gary Illyes repeated this week — at least three times — to SEOs and webmasters who might be concerned about the upcoming switch to a mobile-first index.
“The team behind the mobile-first index wants it to launch this year,” Illyes told our SMX West conference on Wednesday. “We’re still experimenting. We don’t have a timeline. It could be a few months or quarters, but it’s definitely not weeks [away]. Don’t freak out, especially if you have a responsive site.”
Later in June of this year
http://searchengineland.com/googles-mobile-first-index-likely-not-coming-2018-earliest-277074
"Google is “probably many quarters away” from launching its mobile-first index. So said Gary Illyes, Google webmaster trends analyst, during a crowded session Tuesday afternoon at our SMX Advanced conference in Seattle.
“It’s going to be a big change, but don’t freak out,” Illyes said.
SEOs and webmasters have been wondering and waiting for a couple of years now for news on when the mobile-first index will roll out. Illyes wasn’t able to give an exact answer to that question today.
“We don’t have a timeline for the launch yet,” Illyes said. “We have some ideas for when this will launch, but it’s probably many quarters away. Our engineers’ timeline was initially end of 2017. Right now, we think more 2018. ”
The point I am making here is that you need to follow the current Google guidance as Roman mention so that you can rank now. Yes, you will probably need to tweak things once the mobile first index comes out, but until it does and Google then updates guidance on how you should setup the canonical and alternative tags as suggested.
You other option to act on now is to convert to a responsive site, work with one set of URLs and make sure that when your responsive page shows that it passes mobile friendly tests etc. You may end up with a slower page, but you would need to test and see how much of a difference makes. If the page is still fast overall, you should still be good.