ICMP Packet forwarding from WWAN to WLAN fails

We have a project similar to an LTE Mobile Hotspot or MiFi where we use openWRT. We basically use our device as a router where the end client can connect and send data to the cloud. MTU for both WWAN and WLAN is 1500. But we see that when our client connected to the WLAN sends a packet of 1514 Bytes (observed on tcpdump of WLAN), which then forwards it to WWAN where the packet strips down to 1500 Bytes (observed on tcpdump of WWAN), ICMP packet is seen on WWAN that says 'fragmentation needed' but it is not forwarded to WLAN or Client.

Does anyone know why this happens or how can we resolve this?
Appreciate any leads. Thanks in advance!