How can I solve this using bigmod method?

If the series is 1+a+a^2+a^3+a^4+a^5 then this will be equal to (1+a^2+a^4)+a(1+a^2+a^4). How can I apply bigmod here!

BigMod: this is for calculating the mod of a large number like 2^100 or 10^18.

ll bigmod(ll a, ll b, ll m)
{
    if(b==0) return 1;
    if(b%2==1)
    {
        ll p1 = a%m;
        ll p2 = bigmod(a,b-1,m);
        return (p1*p2)%m;
    }
    if(b%2==0)
    {
        ll h = bigmod(a,b/2,m);
        return (h*h)%m;
    }
}