Lots of questions/issues there. First, WRT DDNS, both Starlink and your cellular WANs are behind carrier-grade network address translation (CGNAT.) If you do a search of this acronym here on the Forum and on the internet you’ll “get it.” DDNS will not work. Indeed, this has been discussed here ad nauseam. Essentially, inbound connections are not possible – for reasons well beyond Peplink’s control.
There are solutions to this, the best of which may be to spin up a FusionHub (the 'Solo" license is free from Peplink.) After doing so, one constructs a PepVPN or SpeedFusion tunnel between your router and the FusionHub. Inbound connections would be via the FH. (Search the Forum and Youtube. @MartinLangmaid has a great video on Youtube that really explains this well – few could do better explaining.)
I think the preceding paragraph answers the questions in your 2nd paragraph except for this: SpeedFusion Connect (formerly, “SpeedFusion Cloud” – a much more descriptive and logical name) is an “outbound service” [my term, not Peplink’s.] Inbound connection are not possible. This is an extremely useful tool – but not for inbound traffic.
I submitted a feature request in 2015 requesting the capability to allow routers’ built-in DDNS client to provide updates with the “then best” healthy WAN. (It was also discussed here.) So far as I know this has not been implemented. While it would help many of us yet today this would not solve your problem because you are still behind CGNAT. Even if DDNS were updated inbound traffic would not “get through” in your situation. [Shameless “plug” to ask Peplink to consider this geriatric feature request. ;<) ]
As to your final question, that’s quite easy. Look for “Outbound policies” in your GUI menu. I don’t have a B30 Pro handy but I’d expect to find it at Network → Outbound Policy. This is an extremely powerful (and, I’d venture to say, often underused) too. Check out your firmware manual for details. (You can download it here if you don’t have a copy handy.) If you want to ensure the traffic from a given LAN client always uses a certain WAN this is the tool that will make it happen. Very versatile and works perfectly in my experience.