No, you cannot insert a SIM card directly into this router as it does not have a SIM card slot. However, you can use a 3G/4G USB modem or 4G/5G mobile tethering via the USB port to surf on the move.
I was able to carry out a search and it does indeed look like an incompatibility. Whether it is due to the router, the SFP+ module, a defect, the configuration or the combination of components, it is not possible for us to suggest a specific solution. In principle, I would have the compatibility checked with the manufacturers. I am releasing the question again, as someone may have successfully used this combination, as you have already mentioned, and may know a trick.
I couldn't find a button, but it is possible to manage the entire network using an application on the assus phone (disable wifi, create a guest wifi, etc.).
Yes. You can connect the SFP+ to the router port. But you would need to configure the router to match the requirements from the operator.
I have mine connected to Swisscom and it works flawlessly.
According to the manual, you must open the WAN tab in the router menu and deactivate Dual WAN. You should then be able to assign the port as LAN instead of WAN.
If this does not work, I recommend the following ASUS support. According to my information, this can be used free of charge: Asus Techsupport.
According to my research, it all depends on the routers and their range of functions, where the suppliers ship and may not release all functions. Another option is to ask Swisscom and Sunrise and co which routers you can use yourself, as they have the most up-to-date list and must release these routers. Swisscom should also allow you to use your own router via fibre optics. As for your question, unfortunately I don't know of a reliable and guaranteed solution without bridge mode.