Fortunately, Paragon makes a $15 Mac utility called ExtFS
Where is it available at that price? On the Paragon page it’s listed at $39.95. I also checked whether there was a discount for TidBITS subscribers, but none is listed.
This particular model of Asus router requires the storage drive to be formatted with the older (but still adequate) Linux Ext3 file system, which it can do via its Web-based interface.
One can format an SSD in Ext4 on an Asus router, though admittedly it’s not for everyone. I managed it without any difficulty by following the excellent post: How to partition bigger than 2TB disk with GPT and ext4. I’m running Time Machine on it and it’s been rock solid.