Wow thanks for the quick response 
It gets a little more complicated though...
We need the dynamic SRX to talk to the static SRX, and the static SRX will then initiate the VPN connection.
I think the solutions I've been reading up on so far have been peer to peer. Giving the dynamic SRX a hostname of myjuniper.net and then having some sort of DNS lookup won't work. It kind of needs to be like a Pulse Secure VPN but using the router instead of the software client.
(I really appreciate your advice so far, got my JNCIA, studying for JNCIS, but I've only done a couple fo site-to-site vpns so far )