Disconnected by openvpn server in first two hours

I have setup the openvpn server and client. The client could connect to the server. Then, the server will aways disconnect the client after around two hours(refer to the log of server). And to my surprise, the client shows the connection still on. But I can't send any data from client to server at that time. Here is the config of server(part):

duplicate-cn
keepalive 10 120
nice 3
verb 0
mute 10
reneg-sec 0

Here is the log of server:

Feb  1 07:59:32 OpenVPN server: peer xxx.xxx.xxx.xxx (client) connected - local IP: 10.8.0.2
Feb  1 09:33:34 OpenVPN server: peer xxx.xxx.xxx.xxx (client) disconnected, sent: 55 KB, received: 56 KB
Feb  2 07:59:08 OpenVPN server: peer xxx.xxx.xxx.xxx (client) connected - local IP: 10.8.0.2
Feb  2 09:34:56 OpenVPN server: peer xxx.xxx.xxx.xxx (client) disconnected, sent: 56 KB, received: 57 KB
Feb  3 07:58:12 OpenVPN server: peer xxx.xxx.xxx.xxx (client) connected - local IP: 10.8.0.2
Feb  3 09:58:20 OpenVPN server: peer xxx.xxx.xxx.xxx (client) disconnected, sent: 70 KB, received: 71 KB

Here is the config of client(part):

client
dev tun
proto udp
remote xxx.xxx.net 2222
resolv-retry infinite
nobind
persist-key
persist-tun
auth SHA1
cipher BF-CBC
comp-lzo adaptive
nice 0
verb 3
mute 10

Here is the log of clinet:

2021-2-3 7:58	VPN profile [client] is connecting to [xxx.xxx.net]. (protocol[OpenVPN], IP[10.8.0.2], interface[tun0])
2021-2-2 7:59	VPN profile [clinet] is connecting to [xxx.xxx.net]. (protocol[OpenVPN], IP[10.8.0.2], interface[tun0])
2021-2-1 7:59	VPN profile [clinet] is connecting to [xxx.xxx.net]. (protocol[OpenVPN], IP[10.8.0.2], interface[tun0])