How to correctly set network settings in Asterisk 11 (Centos7, Issabel4, chan-dongle)?

Question

Asterisk

Good time.
We assembled an Asterisk server in the office - a stand-alone machine Centos7, Issabel4, an external device of four Huawei E1550 modems connected directly to the server. The server is connected to the office network, IP phones (grandstream and cisco) are also connected there. The Internet comes to the network through the zyxel cenetic lite 3 router.
The whole thing worked exactly until the moment there was Internet. When it is disabled, it is impossible to connect to the server either via SSH or via the WEB interface, IP phones will not connect either.
Server adapter settings (eth-0)

spoiler

TYPE="Ethernet"
BOOTPROTO="none"
DNS="8.8.8.8"
IPADDR="192.168.0.52"  # ip сервера 
PREFIX=24
DEFROUTE="yes"
IPV4_FAILURE_FATAL="no"
NAME="eth0"
UUID="b3ed8798-f82d-4e62-835d-02e702f66c54"
DEVICE="eth0"
ONBOOT="yes"
PROXY_METHOD="none"
BROWSER_ONLY="no"
IPV6INIT="no"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_FAILURE_FATAL="no"
IPV6_ADDR_GEN_MODE="stable-privacy"
IPV6_PRIVACY=no
GATEWAY="192.168.0.1" # ip роутера откуда приходит интернет

If you comment out GATEWAY at the same time, then the server starts working, but only without the Internet.
The Internet on the server is only used to send mail.
Please tell the newcomer to network affairs how to properly configure the network adapter.
Switching scheme:

I connected another network card to the server and connected the Internet directly to it, like this: eth0

settings :

spoiler

TYPE="Ethernet"
BOOTPROTO="none"
DNS1="8.8.8.8"
IPADDR0="192.168.0.52"
PREFIX0=24
DEFROUTE0="yes"
IPV4_FAILURE_FATAL="no"
NAME="eth0"
UUID="b3ed8798-f82d-4e62-835d-02e702f66c54"
DEVICE="eth0"
ONBOOT="yes"
PROXY_METHOD="none"
BROWSER_ONLY="no"
IPV6INIT="no"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_FAILURE_FATAL="no"
IPV6_ADDR_GEN_MODE="stable-privacy"
IPV6_PRIVACY=no

eth1:

spoiler

TYPE="Ethernet"
BOOTPROTO="none"
DNS1="8.8.8.8"
IPADDR1="192.168.1.59"
PREFIX1=24
DEFROUTE1="yes"
IPV4_FAILURE_FATAL="no"
NAME="eth1"
#UUID="b3ed8798-f82d-4e62-835d-02e702f66c54"
DEVICE="eth1"
ONBOOT="yes"
PROXY_METHOD="none"
BROWSER_ONLY="no"
IPV6INIT="no"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_FAILURE_FATAL="no"
IPV6_ADDR_GEN_MODE="stable-privacy"
IPV6_PRIVACY=no
GATEWAY="192.168.1.1"

The situation has improved, but not completely. In the absence of the Internet, a connection from the network occurs, but with a delay of five seconds. That is, I use putty, he asks for a login, then a pause of five or six seconds, and only then an offer to enter a password. Also, if I call from outside, the phone is beeping for five seconds, and only after that the ip phone starts ringing.
If at the same time I turn off the network with the Internet (ifdown eth1), the connection and calls go through instantly

Answer 1

Try to make DEFROUTE="no", but in general see the settings in /etc/sysconfig/network-scripts/route-eth0

Answer 2

Asterisk without DNS will work very poorly, or not work at all.
SSH connects for a long time, by the way, the same is due to the lack of DNS. Make a caching DNS in your office and the telephony situation will improve.