Nginx + PHP FPM + MYSQL servers going down when too much traffic

We currently have some servers hosting a e-commerce platform in Digital Ocean, we tried to optimize how much we could, but was not enough, site is going down when too much traffic, like 800~1000 users. We currently think its the API that is going down. The error is:

2017/11/23 14:20:51 [error] 26784#26784: *126599 connect() to unix:/var/run/php/php7.0-fpm.sock failed (11: Resource temporarily unavailable) while connecting to upstream, client: {ip...}, server: www.....com.br, request: "GET /....", upstream: "fastcgi://unix:/var/run/php/php7.0-fpm.sock:", host: "www......com.br"

We currently have:

Front-end server:

4 cores, 8gb ram - running nginx 1.10.3 and php fpm 7.0

API server:

8 cores, 16gb ram - running nginx 1.10.3 and php fpm 5.5.9

MYSQL server

MASTER + 8 SLAVES (We don’t think thats the problem), We had some problem before with connection numbers tcp/ip ports (I dont know exactly), but we could’t solve and created slaves for load balance. Servers are running linux ubuntu 14.04

If more information is needed, please ask. We are struggling with this.

Nginx conf:

worker_processes auto;
worker_rlimit_nofile 65536;
worker_connections 2048;
use epoll;
multi_accept on;
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 15;
types_hash_max_size 2048;
server_names_hash_max_size 4112;
access_log off;
gzip on;
gzip_disable "msie6";
gzip_vary on;
gzip_proxied any;
gzip_comp_level 2;
gzip_buffers 16 8k;
gzip_http_version 1.1;
gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;]
fastcgi_buffers 8 16k;
fastcgi_buffer_size 32k;
fastcgi_connect_timeout 360;
fastcgi_send_timeout 360;
fastcgi_read_timeout 360;
fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
client_body_buffer_size    128k;
client_max_body_size       10m;
client_header_buffer_size    1k;
large_client_header_buffers  4 4k;
output_buffers   1 32k;
postpone_output  1460;
client_header_timeout  3m;
client_body_timeout    3m;
send_timeout           3m;

PHP FPM conf:

pm = on demand;
pm.max_children = 200;
pm.process_idle_timeout = 10s;
pm.max_requests = 500;

/etc/sysctl.conf configs:

net.ipv4.ip_local_port_range = 2000 65535;
net.ipv4.tcp_rfc1337 = 1;
net.ipv4.tcp_fin_timeout = 15;
net.ipv4.tcp_keepalive_time = 300;
net.ipv4.tcp_keepalive_probes = 5;
net.ipv4.tcp_keepalive_intvl = 15;
net.core.rmem_default = 31457280;
net.core.rmem_max = 12582912;
net.core.wmem_default = 31457280;
net.core.wmem_max = 12582912;
net.core.somaxconn = 65535;
net.core.netdev_max_backlog = 65535;
net.core.optmem_max = 25165824;
net.ipv4.tcp_mem = 65535 131072 262144;
net.ipv4.udp_mem = 65535 131072 262144;
net.ipv4.tcp_rmem = 8192 87380 16777216;
net.ipv4.udp_rmem_min = 16384;
net.ipv4.tcp_wmem = 8192 65535 16777216;
net.ipv4.udp_wmem_min = 16384;
net.ipv4.tcp_max_tw_buckets = 1440000;
net.ipv4.tcp_tw_recycle = 1;
net.ipv4.tcp_tw_reuse = 1;