Hi good day , i can help , if you see my portfolio , have more design with avr2560 ( same arduino kit ) with SIM modem i can finish in one week .
send full project in Altium sch, PCB, Gerber, material list, 3d model recommend for this procesor Bascom Compiler is easy work and bug free, send little software for this PCB.
if you need any question please send leter over chat.
next if you need full software continue.
have a nice day