Thanks to
https://forum.openwrt.org/t/zyxel-gs1900-10hp-revision-b1-support-openwrt-firmware/131841/32
for putting me on the right track for this problem
Error that is being fixed - running fw_printenv results in:
"Warning: Bad CRC, using default environment"
and not showing boardmodel
Workaround, manually changing /etc/fw_env.config to
"/dev/mtd1 0x0 0x400 0x10000"