The fact that it's 4G / wireless makes me shudder even more. Because it's wireless, all bets are off for latency values
due to weather, interference, LOS, radio reflection / refraction, etc.
Not sure what model netgear you're planning on running, but you should also be aware that home routing gear's CPUs
are scaled for price and NOT low latency en/decryption of VPN traffic, which can be another factor.
As to setting up DDWRT VPN with Astril, did you at least try looking at this page