@AndyHarrowven 

Hi, you need to go to Wireless -> Wireless advanced -> Turn on client isolation, ensure WDS is turned off in Wireless basic, this will stop your devices talking to each other as well so you need to figure out what you need to be available to you but this will make it as secure as it can be.

Next enable the guest wifi and enable -> the 2nd option which  is -> Guest isolate, don't forget to save each time you alter a setting, you may want to restrict their bandwidth as well but be sensible with it.

Personally I'd go get a newer AX router at the very least given what you want to use it for so you and your guests will have WPA3 as an option as well, you should set the guest network to use WPA2/3 mixed or you may have guests that can't connect on just WPA3.

I suggest you have a rolling password on the guest network so you need to change it each time you have a totally new set of guests and use something simple but effective for them like ... ( ANDYFi591?285!wel-come2025 ), this will be simple for them to set up but I suggest you use an alternative version of characters for the password not this one I've just posted, or you can leave it open without a password but that wouldn't be something I'd suggest you do.

Also if the lodges are spaced out and you want the guests to actually be able to use the wifi you're going to need a router with some shout behind it, I'd suggest something that is at least 5400mbps as an 1800mbps model may not throw far enough for the lodges to get a signal, yours is a 300mbps model so it's not going to shout far or very fast.