The router can be operated without a SIM card, but it is primarily designed as a 5G router and requires a mobile phone connection via SIM for Internet access. A direct connection to the WLAN of a campsite as a client (to receive this WLAN and then pass it on to several devices via its own WLAN) is not explicitly mentioned as a function in the available information.
The device supports router and bridge mode, WLAN with up to 64 devices and multi-WAN, but there is no clear indication that it can be used as a WLAN repeater or client for an external WLAN (e.g. campsite WLAN) in order to buy only one access and share it via the router.
It is therefore very likely that the router is not designed to log in via the WLAN of a campsite and then pass this WLAN on to several devices via its own WLAN. Rather, it is intended for operation with its own SIM card and mobile phone connection.
For your plan to buy only one access to the campsite Wi-Fi and then distribute it to several devices via the router, a router with client or repeater mode for Wi-Fi would be necessary, which is not confirmed here.
To summarise: The router requires a SIM card for Internet access and cannot be used as a WLAN client to receive external WLAN (campsite) and distribute it internally.
The FWA510 is cheaper because it is intended for end customers, with free access, cloud updates and better availability. The NR5103 is a more expensive provider device with limited support. Hardware is almost identical.
I was able to do a search and it is possible that the option was moved or even removed after the update. Unfortunately, as I don't have the router in use, I can't test it myself. I am therefore happy to open up the question to those who are already using the router and can share their experiences with us. Alternatively, I would also recommend contacting the manufacturer directly.