GL-MT300N-V2 - VPN server issue

Hi, I'm unable to setup a Wireguard VPN server on my Mango (GL-MT300N-V2) for using my home router IP address from a remote (client) connection.
I have a main router from my ISP to which I plugged the Mango router (WAN port on Mango to a LAN port on the main router).
The Mango IP address in the main home LAN is 192.168.1.55 (shown in the admin console, "Ethernet" tab, gateway and DNS server 192.168.1.1, DHCP).
I well configured the Wireguard VPN server; it is ON listening on the port 51820 (tunnel IP address 10.0.0.1/24); I created a client profile that I used on my client devices. The Peer section of the config file includes my public IP address and port 51820.

On my main router I configured a port forwarding for external port 51820 to 51820 on the Mango (192.168.1.55). I opened the port 51820 on the Mango too.
The port forwarding on my main router works well, as I tested by forwarding 51820 to internal port 22 on the Mango so I have an SSH access from Internet to the Mango. Then I put back the forwarding to 51820 (VPN server).

On my client devices, when I connect the VPN client I expect to have Internet access and the public IP address of my main router.

I don't understand what I'm doing wrong. My VPN tunnel doesn't work, there is no Internet access from my client devices.

Thanks in advance for your help

Did you forward UDP? Your forward test to ssh would use TCP, where Wireguard uses UDP.

Please check How to troubleshoot WireGuard

Yes, UDP is forwarded (see attached)

Thanks for your advice. As far as I was able to check, it seems everything should be ok : network is separate (Mango range is 192.168.8.x and main router 192.168.1.x); I'm not behind a mobile network; port forwarding is ok; dynamic IP address: yes but in practice my public IP address hasn't change since weeks; the port 51820 is not blocked (I tested it); VPN conf seems ok.

AllowedIPs=0.0.0.0/0 is set inside the config?

Yes, it is. In the config file, I have:

[Peer]
AllowedIPs = 0.0.0.0/0,::/0
Endpoint = ...

Can you ping in both directions?

Yes, I can ping the main router from internet, it works. And from Mango any internet host and my router public IP address. But I cannot ping from the main router, I dont have terminal access to it

Any further advice on this issue ?

When I use a VPN client within the internal LAN (of the Mango or the main router), it works.
But obviously I need it working when the client is on internet...
First thought is that the port forwarding doesn't work on the main router, but when I test (forward the port 51820 to other internal ports on Mango like 22 (ssh) or 80 (http)) I can reach the Mango from Internet.

You tried your VPN from the internet? Like a hotspot or something?

yes, with a mobile hotspot from my mobile phone