ASUS Router [AC53U] is blocking smb access from remote source [outside LAN]

[mannok]

I found that my AC53U is blocking SMB access from remote source. Is there any way to re-open it to outside?

My situation:

My Home: AC53U

• LAN Network: 192.168.3.0/24
• USB HDD plugged in, SMB enabled, FTP enabled
• Acts as a VPN client of my mum's home [site-to-site]

My Mum's Home: AC86U

• LAN Network: 192.168.1.0/24
• Acts as a VPN server [sit-to-site enabled] [route pushed]

My PC:

• IP: 192.168.3.11
• Connecting directly to AC53U via WIFI
• CAN access to both FTP and SMB of AC53U

My Android:

• ISP IP: 114.x.x.x
• Logged into AC68U's VPN
• CAN access to FTP of AC53U by 192.168.3.1
• CANNOT access to SMB of AC53U

From the above observation, it seems that AC53U is blocking the outside traffic from accessing it SMB. Any suggestion to open the SBM service to outside? Thanks in advance.