This is not a question, but simply to share my experience, in case it is of use to others in a similar situation.

I live in a semi-rural area with Spark ADSL+ on an unlimited plan. Typical speeds are at best 5-6 Mbps down and 1 Mbps up.  An upgrade to VDSL is unlikely, let alone fibre.   RBI Wireless 4G is now available in my area, but it would be very expensive to put all my traffic (hundreds of GB a month) through that.  4G speeds are up to 35 Mbps down, and 25 Mbps up, using a roof antenna.

I have retained the Spark ADSL plan, and added a $99 per month RBI 4G plan (I'm using Ultimatebroadband, who resell it from Vodafone I think) which gives me 80 GB per month, plus an additional 50 GB off-peak (midnight to 6 am).

I purchased a TP-LINK TL-R470T+ dual WAN router ($69 from Elive) and plugged an Ethernet cable from the Huawei 4G modem into WAN1, and from the existing ADSL modem into WAN2.  Each was set up with dynamic IP, and this setup worked fine out of the box. Both the ADSL and 4G modem/routers serve DHCP on the 192.168.1.x subnet, while the TP-LINK is on 192.168.0.x

I'm using the TP-Link as a DHCP server, with a LAN connection out to the existing home wireless network.  All known devices have been assigned static IP addresses (based on device MAC numbers) and then I am using Policy Routing based on their IP numbers, so that only those devices that need it or 'deserve' faster speeds (e.g., phones, tablets, and Apple TV) get to use *both* WAN1 and WAN2, while computers which do lots of downloading are left on WAN2 (ADSL).  Eventually, as (hopefully) the price of RBI 4G Wireless comes down, more of the devices can be moved to take advantage of that, or maybe I can drop the ADSL altogether.

Works a treat, and I've been delighted with the TP-LINK TL-R470T+ router to combine the two WANS and route traffic appropriately.

(One downside is that Unblock-US gets a bit confused, because traffic requests appear to be coming from two different public IP addresses. That requires me to reenter my email and password from time to time.)