… which IP address does the outside world see? The static one or the 4G IP?
If you visit a site which reports your public IP, it sees whichever connection the load balancing router used for that traffic on the way out.
The algorithm will usually have options for varying degrees of stickyness. For example traditionally when establishing a TLS connection (e.g. https) it would send subsequent related traffic out through the same link until the client closed the connection. The way websites and online services use CDNs these day, it probably isn't that clear cut.
If you are thinking about port-forwarding, while you can add the same rules to multiple connections, it will likely only make sense to come in on the static IP connection. Even with dynamic DNS it is not guaranteed that the the 4G/5G connection will always have a routable public IP for inbound connections as compared with an IP shared amongst other subscribers.
If either ISP supports dual-stack w/ IPv6 your options change again.
prlzx on Zen: FTTC (VDSL) at ~40Mbps / 10Mbps
with IP4/6 (no v6? - not true Internet)
Edited by prlzx (Sun 31-Jan-21 12:53:53)