More than excellent Dr Amani.
As you have already mentioned, atenolol is a cardioselective beta-blocker.
But all cardioselective beta blockers share the same side effect profile
as propranolol ,the non-selective beta-blocker, to a varying
degree,specially at high doses.
The first line of management here,surely after exclusion of other causes,
is to substitute the antihypertensive drug rather than addition of other
drugs such as Sildenafil ( Viagra ).
We can shift to ACE inhibitors such as captopril, or calcium channel blockers
such as amlodipine.
Thiazide diuretics wouldn't be a wise choice, they are known to cause
impotence(erectile dysfunction) themselves.