Sometimes I can access some.web.site but not some.other.web.site, getting ERR_NAME_NOT_RESOLVED for the latter

Currently my internet provider is Lebara, in UK. Previous days internet seemed to work just fine. Today I've been having a recurring issue which affects both laptops and smartphones connected to the WiFi (fwiw, I don't have a wired connection, nor an ethernet cable to try connect via that to the router), where I can access some sites but not others, and I see absolutely no commonality between the two categories. See below for a few examples. I say recurring in the sense that when it happens, I can just disconnect my laptop from the WiFi, connect it to, say, the smartphone's data connection, an

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