Best OTP 2FA with Free Radius and AD

I have freeradius working with Activedirectory to authenticate users on a Fortinet Gateway. I'd like to add OTP so users can use Google Auth, Authy, Microsoft Authenticator or their favourite OTP software. What are people currently using and is there a recommendation for something on the Freeradius server? I've searched for options and getting a million answers only to find after going down the garden path there are issues with how codes are issued. Just after some tips on what people have used.