Your nice pic does not show that, but I suspect that this
Means that one of the devices (10.54.53.240) on the subnetwork with satellite router (10.54.53.250) and RTU127 (10.10.53.240??) sends SYN to the 10.15.1.80 (master switch, or something connected to master switch), you seen this SYN packet on the wireshark PC connected to BMD_SIGS_SW07, but do not see ACK packet returning back from 10.15.1.80 to 10.54.53.240?
Where’s W5300 device on the picture? Instead of trying to guess what is going on I propose (again) to insert network bridge PC in between of W5300 and other network, and perform capture there when you get symptoms of the problem to prove that:
- W5300 still receives SYN packet to reply to;
- W5300 does not respond with ACK packet if it receives this packet.