Thanks for input anyway. In my case, the ISP docs have a (very brief) section about using own router:
With a NTT FLET'S connection line your broadband router will need to be PPPoE (PPP Over Ethernet) and PAP (Password Authentication Protocol) compatible, unless the line is FLET'S Hikari Next in which case your router will need to be PPPoE and CHAP compatible. If you are unsure, please refer to the manufacturer's documentation for details.
When using a broadband router that has not been supplied by NTT, you may encounter problems such as not being able to open some Web pages or Web pages opening slowly. In these cases, please try changing the Maximum Transfer Unit (MTU) setting in the broadband router to 1454. By default the MTU setting for broadband routers not supplied by NTT is 1490, a value of 1454 is more compatible with NTT's FLET'S network.
I already found the MTU setting in EdgeOS GUI screenshot, but still unsure about the PPPoE/CHAP support.
