Why do you have both an NLB and an ALB?
AWS Application Load Balancer has security groups (documentation link). You can use that to limit which IPs traffic can from from. That relies on NLB not messing with that information.
The AWS Network Load Balancer works at Layer 4, and according to Wikipedia "Class 4 is closest to TCP". I wonder if NLB doesn't change the source IP - hopefully someone else who's more familiar with this can answer / comment, it's been a long time since I learned the OSI model. If so this would work well with the ALB security groups.
Another approach is that Securi most likely sets the X-Forwarded-For header. In Nginx you can use the "real_ip_header" command to say "for the IPa addresses I specify (ie that belong to Securi) the original IP address is in this header". eg:
To be careful of is that the AWS ALB might also set that header, hiding what Securi sets.
Nginx Allow / Deny
Once that's done you have another method using Nginx that lets you block all hosts other than those you allow.
allow 184.108.40.206; # Allow a single remote host
deny all; # Deny everyone else
If you can simplify your architecture you can probably find a simpler solution.