Ubuntu L2TP Server not allow to connect multiple devices

Till today, this website showed me some answers that solve the problem. But it seems I couldn't find the correct answer to this problem.

I have a project for a VPN network for remote service purposes, the network will not be used constantly. I developed C# software for windows clients to set Built-In Windows clients to connect to the company's on-premise server.

The server runs Ubuntu 20.04LTS and I set up Strongswan VPN has 2 clients on it.

Client A and Client B can connect L2TP server without any hassle. Any I can ping to Server from both Clients. When I try to connect with both A and B clients, the server just allows a single device to connect and I can ping the server just a single Client.

What I need pinging from Client A to Client B. I felt can't set ufw firewall or iptables, but not sure where to follow.

Configuration below:

/etc/ipsec.secrets

192.168.0.17 %any : PSK "xpasswordx"

/etc/ppp/chap-secrets

userclientA * xpassx 192.168.30.10
userclientB * xpassx 192.168.30.11

/etc/ipsec.conf

config setup
conn L2TP-PSK-noNAT
        dpdaction=clear
        authby=secret
        auto=add
        keyingtries=3
        ikelifetime=8h
        keylife=1h
        keyexchange = ikev1
        ike=aes256-aes192-aes128-sha384-sha256-sha1-modp2048-modp1536-modp1024!
        type=transport
        left=192.168.0.17
        leftprotoport=17/1701
        right=%any
        rightprotoport=17/%any

/etc/xl2tpd/xl2tpd.conf

    [global]
    ipsec saref = yes
    saref refinfo = 30
    ;debug avp = yes
    ;debug network = yes
    ;debug state = yes
    ;debug tunnel = yes
    [lns default]
    ip range = 192.168.30.2-192.168.30.254
    local ip = 192.168.30.1
    refuse pap = yes
    require authentication = yes
    ;ppp debug = yes
    pppoptfile = /etc/ppp/options.xl2tpd
    length bit = yes

/etc/ppp/options.xl2tpd
    
    logfd 2
    logfile /var/log/xl2tpns.log
    refuse-mschap-v2
    refuse-mschap
    ms-dns 8.8.8.8
    ms-dns 8.8.4.4
    asyncmap 0
    auth
    crtscts
    idle 1800
    mtu 1200
    mru 1200
    lock
    hide-password
    local
    #debug
    name l2tpd
    proxyarp
    lcp-echo-interval 30
    lcp-echo-failure 4

iptables -L

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     udp  --  anywhere             anywhere             policy match dir in pol ipsec udp dpt:l2f

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

/etc/sysctl.conf

net.ipv4.ip_forward = 1
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.icmp_ignore_bogus_error_responses = 1