Depending on your ISP connection method this could be completely normal.I'm a bit confused how I access 192.168.100.1 from my LAN-based computer... to me it seems like computer -> Router LAN -> Router WAN -> Modem IP... but router WAN is that 24.115.xxx.xx address so I'm just not comprehending.
The longer explanation:
I'm in the UK where the use of PPPoE to connect users and properties to an ISP backhaul is commonplace and, for ISPs using Openreach's UK network infrastructure, it is mandatory.
For my home network my router's WAN port is connected to my modem, which acts as a bridge. The actual pathway for the PPPoE from modem to the router WAN is just a regular LAN that has the PPPoE as a virtual pipe within it. Or from a different perspective the modem's PPPoE WAN comes wrapped in a regular LAN.
This LAN containing the WAN PPPoE does not need a network address and in that case the only IP associated with is would be the external WAN IP(s). However, in that case there would be no internal way to communicate to the modem itself for settings and alike as there is no way to route to it (a very common configuration in the UK).
Once you give the LAN containing the WAN PPPoE a network address space (like in your case and in mine) and both the router the modem are assigned to that LAN, then you can route to the modem itself from the internal LAN to access modem controls, settings, data etc. This would give both a routable LAN IP and WAN IP to the same physical port, as you appear to have.
It also gives a potential failure route where either WAN or LAN IP is working but the other is not. It could even be the source of your problem when you switched ISPs and that new ISP used a new modem or new settings within it.
Apologies if I have over or under explained things as I don't know your level of knowledge.
[Edit: I also missed the explanation from @Gerard above when typing this.]