Interesting question. In the first case, where you install your own build from unmodified source code, although AGPLv3.0 still allows discontinuing support, I see no explicit carve-out in the licence to restrict network access.

However, the AGPL comes with no right to such network access to begin with. Permission to access the network would usually come separately from the AGPL; I suppose you could potentially bundle it as an additional permission under section 7, but I don't think Bambu is doing that.

To take it a step further, even if you use the latest official software, installed by the vendor (and not by you), they can still refuse you access to their network. That might violate some other agreements or laws (e.g. contract to provide a service), but it does not violate the AGPL itself.

What they cannot do is prevent you from running your modifications on your hardware.