Upgrading from Core 174 to Core 181 involves several rounds of updates using pakfire, each introducing its own set of changes and potential variables to your system. Under these circumstances, troubleshooting can become quite complex. As a general recommendation, it’s better to apply updates to your IPFire system regularly. Doing so not only simplifies the process of reverting to a previous version if an update adversely affects your firewall’s functionality, but it also ensures that your system stays up-to-date with the latest security-oriented changes in IPFire.
EDIT: Due to my high level of caution when performing updates, I always maintain a cloned copy of my internal hard disk, synchronized to the state before each update attempt. If something goes wrong, I have the option to swap the hard disk and restore the system’s previous state. I then address the problem at a convenient time.
Too many variables involved to guess. Firmware issues with the network card or MTU (Maximum Transmission Unit) negotiation issues would be my first guess.