carefully check the inside of the tire for any punctures or anything sharp that could have punctured the tubes. if you don't find anything that could of caused the flat then more then likely the new tube either got pinched by the rim and tire wall somehow. which with the rims and tires they use on the ancheers they can sometimes be a bit tricky to get the tire and new tube back on the rim where the bead of the tire isn't fully seated unless you put a tiny bit of air in the tube before putting the tire back on the rim then after. go around the entire tire and squeeze it on the sides to put the tube away from the rim so it doesn't get pinched between the tire bead and rim. Then when you inflate it keep checking all the way around on both side of the tire to make sure you don't have any bulges on the sides were the tire is going to try and come off the rim. if you do let some air back out and start over with pushing the tire bead back so its even all the way around and then inflate again. also be sure to also inflate them to no less the 45 psi or else you risk the tire coming off the rim or a pinch flat if you hit a big bump.
I went threw the very same thing last weekend with my vivi bike which are exactly the same bikes and tires as the Ancheer. and i got a flat from hiting a hard bump on a trail. then luckily a friend had a new tube and i got it on the bike but all I had was my dinky crappy hand pump i didn't get it pumped up all the way to the recommended 65 psi and I might not of had the bead set right all the way around and I got 2 miles into my 18 mile return trip home and bam i got a flat again. Then after getting a ride home and changing the tube once again. and not getting the tire fully inflated to the proper psi i was going to ride it down the street to use the air hose at the gas station and I didn't even get one full turn of the tire before it was already coming off the rim. (luckily it didn't pop a 3rd tube) so I put it on once again being super carful to check that both beads were seated properly. pumped it up as hard as i could get it and then was able to carefully ride to the gas station avoiding any big bumps in the road and then was able to fill it properly so it was nice and hard and both beads seated well on the rim. and have been good to go. BUT dam them tires are a real pain to get on right. and it's almost as if they are a tiny bit big for the rim so they fit really loose till they are fully inflated. someday i will upgrade them to better tires but being they have a lot of tread on them and are in good shape I'm just going to be careful to always make sure I won't let the psi get too low and i also now have flat out tire sealant in them so they won't leak from any of the tiny pores in the rubber and it also can self seal up to a 1/2 inch hole.