We had some troubles with the distribution. Some of the merchants who have upgraded to 220.127.116.11 in Merchant wave were able to upgrade to 18.104.22.168 which in Developer wave. As soon 22.214.171.124 also in Developer wave even with Merchant wave in store's settings they saw the ability to upgrade to 5.4.0.x. We fixed it.
Here is changelog for the latest 126.96.36.199 in Developer wave: https://devs.x-cart.com/changelog/188.8.131.52_-_25_Jul_2019.html
Could you please confirm that you're not seeing the ability to upgrade to 184.108.40.206 while your store in Merchant wave?
For productions stores on XC 5.3.6.x I recommend to wait until September 2019.