Hello  @BernardT 

•  You could consider a Access Point that support Range Extender / repeater mode like TL-WA1201  or TP-Link range extender, and this requires RE receive strong and stable wifi connection from the main router. 

Topology: Huawei 4G router (1st floor)<Wireless>AP  working in Range Extender/RE  at a proper location among 1stand 2nd floor)<Ethernet> BPL product  (2nd floor)-wired/ wifi network device

• Range extender may not a perfect one for different floors as signals on vertical level is usually not strong as it on vertical level, you may consider swap the Huawei mobile router to a 4G Desktop router which has Ethernet ports,

Topology: 4G desktop router (using SIM card) on 1st floor-<Ethernet> AP/Router on 2nd floor

Hope this could helps.