I have the same issue with a ZTE ZXHN H298N router. I can successfully connect a USB stick or drive in the USB port using Ftp://192.168.1.254 but get read only access.
Like yourself I don't have the SAMBA option in my router, it doesn't connect using SMB and I can't find any option to enable file sharing.
Did you manage to resolve your problem if so I would be interested to hear what you did.
I have tried ZTE support in the US but they don't handle this product, also tried the UK but got routed to their India call centre and was told to contact ZTE in China. I even registered on ZTE's support website in China and got access but no permissions to access any thing and even a search on their site against ZXHN H298N came up with nothing. Can anybody help!