Simply by adding a /32 route to the address of the RADIUS server via WAN 1 gateway to all routing tables, so regardless which routing table the “bonding” (actually, it’s most likely load distribution) chooses, the packets to the RADIUS server will always go via WAN 1.
It doesn’t work this simple. The regular routing only takes into account the destination address. So without adding a routing rule or a mangle rule that would order which routing table to use based on the source address, forcing a particular source address is not sufficient to make a packet always leave through the interface to which that source address is attached.
And for this particular case, a /32 route seemed simpler to me than a dedicated routing table and a corresponding routing rule. But from 4 routing tables up, adding a 5th one this way requires less configuration rows than adding the same route to all the other tables.