After the number of pptp connections exceeds two, the later connections cannot access the public network.

IP forwarding is enabled for server deployment pptp,iptables.
after the completion of the dial-up connection of other computers, the first two are normal, but it is found that the computers connected behind cannot access the external network.
I am a rookie in the network, so I would like to ask you what the situation is.

the following is the deployment process.
yum install-y ppp pptpd

remove the gateway and dial-up address fields from the logout configuration file
vim / etc/pptpd.conf
localip 192.168.0.1
remoteip 192.168.0.234-238

configure DNS
vim / etc/ppp/options.pptpd
ms-dns 223.5.5.5
ms-dns 223.6.6.6

configure account password
vim / etc/ppp/chap-secrets
123 pptpd 123 *

set maximum transport MTU
vim / etc/ppp/ip-up
add
ifconfig ppp0 mtu 1472

modify the kernel to allow ip to forward
vim / etc/sysctl.conf
net.ipv4.ip_forward=1

sysctl-p overload

iptables enables ip forwarding
iptables-t nat-A POSTROUTING-s 192.168.0.0 MASQUERADE 24-j MASQUERADE

restart
systemctl restart pptpd