What diffrences bewteen dectphone connected to modem and connected to router (the tel indicator solid even no phone plugged in) . I noticed that some user dectphone is connected to router and some is connected to the modem.

I also noticed that unifi fiber user dectphone is connected to the huawei modem (ONT) and Maxis fiber user's dectphone is connected tp the tp link (ArcherC5v) router.

May i know what diffrences bewteen connect to router and connect to modem. Is it set by the ISP or we can change ourself?

Probably the wrong thread but honestly there's not difference
Unifi puts it on the modem because they have full control over it, and allows easier replacement of own router for their customers

Maxis isn't allowed to use those ports on the modem, so they get a router with telephone ports and connect there instead.
In some rare cases you could get maxis voip working on the modem

You could attempt to change it (there are a few threads here) but for the most part you don't need to or can't in some cases

Also, try calling your own number, you should see the Tel light blink
On means connected to the network, blinking means incoming call, off means not registered to the network