I have a Vigor2860ac with a Huawei 4G router on WAN2 ethernet and a Sky BB connection on the DSL(WAN1).
I have both these connections as the BB connection is slow in my rural location.

I have some devices, CCTV, boiler system interface etc. that need to be accessed remotely using a fixed IP/dynamic dns which is not possible via the 4G modem connection due to a shared external IP.

I use a dynamic dns on the DSL WAN1 connection and port redirection to facilitate this remote access. It works OK but as the upload speed on the DSL is very slow here at only 0.6mbs, I wondered if there was anyway once the initial remote connection is made to say my CCTV on DSL WAN1 via the dnynamic dns, if somehow the Vigor router could then switch over or route that sessions remote requests to make use of my 4G connection which has speeds of around 19Mbs. I am not sure if this is even possible or if it is, how to configure the router to do this. any help would be appreciated. Thanks

I think the solution to this (if there is one), will be specific to the services you're trying to access.

I'm not familiar with commercial 'CCTV', but two 'Boiler System Interfaces' I am familiar with (Netatmo & Wiser), both make periodic outbound connections to cloud servers, to collect their latest instruction and pass back their latest data/status reports. (Ditto Sonoff/EWELink devices). In these instances, "Route Policy" on the Vigor, would be all you would need (having figured out who they're talking to and who's doing the talking.

But if the device had just a simple Web Server interface (for example), this wouldn't work - the reply to incoming request would take a different route back, and the client probably wouldn't recognise it as such (since it comes from a different IP address).

I'm thinking an outbound VPN connection from the Vigor, via 4G could be the answer - but then the question becomes: "what is the client? ... could it be a VPN Server? ... or could it connect to some mutual VPN Server that the 2860 connects to?"

I don't think it's straight-forward :?

Getting a real-world IP address for your 4G connection would be the easiest solution. People on here seem to have managed that - at reasonable cost. I'm not explored it myself.