your signals "seem reasonable"... that is layer-1, lwIP starts at layer-3.
Do you know if your messages actually go out of your hardware ?
Have you tried sniffing with wireshark ?
Above your hardware there is:
Your driver, the layer-2 code, in lwIP terms: "the netif". This has been written by someone else, or you, and although it follows some guidelines, does not belong to lwIP. There is some flag in the driver code that enables broadcasts (NETIF_FLAG_BROADCAST) to be received, but I guess this does not apply (I'm not fluent in DHCP and I don't know if this flag also affects sending). Besides that, your chip might need some initialization too.
Perhaps if you attach a wireshark capture file we have less to guess ?
How do you know the driver works ?
And gluing all together is:
Your port, the main loop or thread, and some single-threading requirements at the low-level functions you must obey. Do you ? This includes not calling lwIP code from main loop and interrupts at the same time (NO_SYS=1) or from different threads with an RTOS, except for the higher-level functions (socket and netconn), which is not the case for DHCP.
How do you know the port works ?

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