library architecture convertion and skipping incompatible

My Problem: I have a custom library which i need to compile my programm. However the linker does not accept the library as compatible, skips over it and thus fails. Other questions try to link a 32bit and 64bit architecture/library (or vice versa). My library and machine architecture however are both 64bit. The library contains several object files which all have the same file format which i returned with:

objdump -f libmylib.Linux64.a

In archive libmylib.Linux64.a:
obj1.o:      file format elf64-little
architecture: UNKNOWN!, flags: 0x00000011
HAS_RELOC, HAS_SYMS
start address 0x0000000000000000

obj2.o:      file format elf64-little
architecture: UNKNOWN!, flags: 0x00000011
HAS_RELOC, HAS_SYMS
start address 0x0000000000000000
[...]

My guess is the problem originates from the unkown architecture and different file-format, apparently being not compatible with my main.cpp.o files.

My Question:

is it possible to convert the library to the file format:

elf64-littleaarch64

and set the architecture to

aarch64 

(that is the format of my main.cpp.o)? If so how would i do that? I tried extracting the object files and rebuilding the library with

ar -qc --target=elf64-littleaarch64

without any success.

Or do i have to set the proper flags in my CMakeList.txt and if so what are they?

Thanks in advance, any pointing in the right direction or help would be really appreciated