Permission issue preventing increase in Open Files Limit in Ubuntu

Question

My Digital Ocean droplet's ulimit -n yields 1024. Now it's a dedicated machine with Ubuntu 16.04, 32GB Ram, 16 Cores. I want to crank it up to 65535.

The problem:

I followed a bunch of steps to configure this (given at the end). However, I run into a permission issue. Specifically:

1. Nginx error.log in /var/log/nginx/ shows: setrlimit(RLIMIT_NOFILE, 65535) failed (1: Operation not permitted)

2. auth.log in /var/log/ shows: main-app sudo: pam_limits(sudo:session): Could not set limit for 'nofile' to soft=65535, hard=65535: Operation not permitted; uid=0,euid=0

I am scratching my head as to how to solve this. Can an expert show me the way?

---

Background:

There are several guides available across the internet for increasing ulimit -n (and many answers on stackexchange, but none that appear to resolve this permissions issue). The steps I followed were:

i) Added fs.file-max = 65535 in /etc/sysctl.conf.

ii) Ran sudo sysctl -p thereafter.

iii) Added the following lines in /etc/security/limits.conf:

* soft     nproc          65535
* hard     nproc          65535
* soft     nofile         65535
* hard     nofile         65535
root soft     nproc          65535
root hard     nproc          65535
root soft     nofile         65535
root hard     nofile         65535

iv) Added session required pam_limits.so to /etc/pam.d/common-session.

Logged out and back in. Tried rebooting and powering down and up too. ulimit -n won't budge from 1024.