Two-factor authentication when your device is not present
Is there a way to still get into iCloud to activate Find my iPhone?

Yes, you can login to iCloud to activate Find My iPhone. You can get a verification code via SMS or voice call on your trusted phone number.
Refer to this Apple Support document:

https://support.apple.com/kb/HT204915


What if I can't access a trusted device or didn't receive a verification code?
If you're signing in and don’t have a trusted device handy that can display verification codes, you can have a code sent to your trusted phone number via text message or an automated phone call instead. Click Didn't Get a Code on the sign in screen and choose to send a code to your trusted phone number.