Hi everyone.
I would like to give my recomendation if I may.
1, No builder who deserves to be paid will call mortar or concrete, cement. Cement is the powder used to make mortar or concrete.
2, Do not lay 50mm of mortar if you have no aggregate, In fact dont use aggregate or 50mm of mortar at all.
3, I am aware that you are not in Australia but as a guide, in Oz, buiding regulations state that for a balcony you must have a fall of NO LESS THAN 1:90. That means that for every 90mm away from the highest piont your floor must fall 1mm. I think for Thailand given the rainfall this is a good minimum.
4. The finish that I can see in the photo is keyed well for using mortar at no greater than 10mm. Place a straight edge on the floor in various places (or wait for a bit or rain) and identify your low spots and thier depths. Laying tiles with mortar is somthing that Aussies stoped doing in the 60's.
5. Look around and try to find a product called level quick or simmilar, google.
Whithout knowing how deep some of the low points are it is difficult to get you out of them. Just dont use mortar too thick, too wet or too dry.
ps. seal your floor with a slurry of cement powder and water first, immediatly before tiling, just to give your floor some moisture and help the bond, PVA like cemstick or simmilar is advisable too if you can find it.