Solved - it seems like the first of these routers are end of life (ZTE MF285) but newer ones don't have unreasonable locking so if the router is still good, and you own it, you can switch providers without too much complexity.
Does anyone know if DStv SIM locks / carrier locks their ZTE fixed LTE routers?
And if they allow customers who are paid up to change SIMs. Can't find any info in their preferred channels nor would customer care answer me.