Than when you come back from reservaist, you received a notice
from them that you got sacked.
You mean you got a termination letter (directly), or was served
notice first?
Yes, your boss can train your replacement in your absence --
although rather unethical of him -- but, under the law, he cannot
sack you "immediately" unless it's for misconduct (must formally
conduct an inquiry). Otherwise, the notice period, if it's not
specified in your job contract, is usually 1-2 weeks (employed less
than 5 years; I forgot the breakdown) or a month (5+ years). But
only one day's notice is needed if you've only worked for less than
26 weeks.
If you think you're unfairly dismissed -- eg. your boss had
approved your release for ICT in the first place, despite you being
a newbie employee -- then, yes, you can seek legal recourse by
appealing to MOM within one month of the sacking. They'll
investigate &, if found guilty, your boss must either reinstate
(& pay back lost wages) or compensate you. Don't be afraid
either way: he'll have to show evidence to the ministry &,
ultimately, their verdict shall be final & legally
binding.