Hello@Wipkinge 

Thank you for writing a very detailed and clear post!

May I know when issue happen, how about the Led Status on RE450? 

1. To troubleshoot RE instability: 

If  the Wi-Fi LEDs are On the extender, you could follow Case1 in below thread, set up a static IP for range extender and configure RE DHCP settings for extender and see if that  help improve the stability. [Troubleshooting] Range Extender Keeps Losing Connection

2. Another thing that may affect your IP reservation set-up :In Extender mode, RE works as a proxy,  it will replace each of its clients’ MAC address with a virtual MAC address generated automatically by Range Extender. Thus, on router web page you will see the virtual Mac address of device, you could set up IP reservation for Miner's virtual Mac address not real Mac. 

To avoid the virtual Mac, you could use RE as an Access Point (wired to main router):

How to setup Access Point Mode of Range Extender?

Hello @Wipkinge  Sorry for the late update, Extender itself is not a NAT device like a router and should not block any port or service.

So far I could not think of settings on extender that may cause it, you may contact Helium Miner support to confirm and get some more professional suggestion: 

1. If "symmetric" NAT type on miner will affect Miner performance 

2. What settings that could cause a "symmetric" NAT type and their suggestion.

May I know did the Miner show NAT type as "symmetric" before you disable RE DHCP?  For a test you may turn on RE DHCP and set an small IP range on RE DHCP as the above thread I posted and see if that helps.