I have read through a number of articles about how to handle failover internet connections. However I still have 2 concerns. From what what I have tested and gathered with reading:
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DSL,Cable and Fiber - You do NOT need a static IP from them, even if you did have one you still have to set the Asterisk SIP Settings > Chan SIP Settings > Dynamic IP and set a DDNS name (you can even use the DDNS service built into SysAdmin Pro) Obviously you want a fast DDNS Updater (either in FreePBX or on your Router) and you want a fast Dynamic Host Refresh time in the Asterisk SIP Settings so it can happen very quickly.
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4G Modem - You MUST have a Static IP address because most if not all of 4G providers double NAT your connection by giving you a private IP address NAT’ed through one of their Public IP’s and therefore SIP traffic will never traverse without a VPN to your SIP Trunk provider. Even with a Static IP you must still do the DDNS setup as mentioned above
So far am I understanding all of this correctly? ISP Failover seems to be somewhat of a mystery for the most part when it comes to asterisk. lol
Now if all the above is true… that Dynamic Host setting is ONLY under the Chan SIP Settings. Does it ONLY apply to ChanSIP or does it apply to the Chan PJSIP as well?
I have been trying to wrap my head around all this for a few months now I never seem to get it. I have seen others post about the router failing over and it “just works” which I sorta want to call BS on because I work with Sophos UTM’s which have built in WAN failover and sure i can get everything in the network to work except FreePBX being able to call out until I manually hit the Detect Network Settings under General SIP Settings and Apply Config.
Am I understanding all this correctly or am I just rambling nonsense. 
I would like to have a complete article here so future people do not have questions about a simple fail over setup. Its something they will understand.