Best OTP 2FA with Free Radius and AD

[Matthew Beechey]

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.