Can't upgrade Actiontec MI424WR rev. D

I'm having problems upgrading my rev. D from 15.05.

Trying to write zImage from any later version gives the following error...

root@OpenWrt:/tmp# mtd write lede-17.01.6-ixp4xx-generic-zImage  linux
Unlocking linux ...

Writing from lede-17.01.6-ixp4xx-generic-zImage to linux ...  [e]Failed to erase block
root@OpenWrt:/tmp# mtd write openwrt-18.06.2-ixp4xx-generic-zImage linux
Unlocking linux ...

Writing from openwrt-18.06.2-ixp4xx-generic-zImage to linux ...  [e]Failed to erase block

I was fortunate to be able to write 15.05 back ok...

root@OpenWrt:/tmp# mtd write openwrt-15.05-ixp4xx-generic-zImage   linux
Unlocking linux ...

Writing from openwrt-15.05-ixp4xx-generic-zImage to linux ...

Any ideas?