The AGND and AVCC pins on the W5500 are for the ethernet drivers and/or receive amps I think. Make sure the analog supply is separated from the digital with a suitable ferrite bead(s) to keep high frequency noise from it - AVdd needs good decoupling according to the reference schematic.
Reference schematic for W55000:
https://wizwiki.net/wiki/doku.php/products:w5500:refschematic