Quote:
Originally Posted by a1call
A 1.5 k dd number should be matter of hours(if not minutes) with primo.
You will get a certificate if prime. Don't necessarily give you the factor though.

You should reread OP: the question is not whether 1520digit n is prime (he says 11 is a factor, which should have been a hint...), but whether 10^n +7 is prime.
Consider that Prime95 tests numbers of magnitude 10^{8 digits}, or the very smallest 10^{9 digits}; then consider how long a prp test would take for 10^{1520+ digits}.
Finding a factor to show compositeness is OP's only hope.