TFTP server on WAN. LAN Unable to Download File

I am using OpenWRT 24.10.1. I have TFTP client on LAN and TFTP server on WAN. When client makes a request to download the file, server receives the request and responds with first block. The block is also received on WAN. However the packet is not transferred to TFTP Client on LAN. In case, I test it with atftp client installed in OpenWRT, I am able to download the file in OpenWRT terminal.
I captured the log using TCP capture and here is the summary-

I am getting following log when a request is made by tftp client on IP 192.168.1.191 to 192.168.20.8. The log is captured by wireshark on the router with IP aadress 192.168.20.5 on WAN side and 192.168.1.1 on LAN side. The TFTP server 192.168.20.8 is located on a server on WAN side. I see that firewall/ NAT is doing it's job correctly when data is sent. But when TFTP server responds, the packet arrives on WAN but it is not transferred to client on LAN. What configurations I have to do on OpenWRT based router working on OpenWRT version 24.10.1 to make this TFTP transfer work correctly?

472	56.840564	192.168.1.191	192.168.20.8	TFTP	67	Read Request, File: KERNEL.IMG, Transfer type: octet
473	56.840564	192.168.1.191	192.168.20.8	TFTP	67	Read Request, File: KERNEL.IMG, Transfer type: octet
474	56.840728	192.168.20.5	192.168.20.8	TFTP	67	Read Request, File: KERNEL.IMG, Transfer type: octet
475	56.897814	192.168.20.8	192.168.20.5	TFTP	564	Data Packet, Block: 1
477	56.906792	192.168.20.8	192.168.20.5	TFTP	74	Error Code, Code: Not defined, Message: Undefined error code