WIFI Calling Preferences Being Forced to Cellular Preferred

I am working with T-Mobile corporate to try to resolve/mitigate my issues. Like most here, I have the Wi-Fi optimization issue, switching from Wi-Fi to cellular, even though I am just feet away from a very good ASUS router. My cellular signal strength varies from 2-3 in the best location to 1 or none in other areas. I want to be able to take my phone anywhere in my house and not lose a call. This originally worked great with Ethernet connected CellSpot routers, but died after a software update. Many updates later and still doesn't work reliably. I since upgraded to ASUS RT-AC86U routers, hoping to have better connectivity and reducing the impact of leaving T-Mobile. Didn't help my Wi-Fi optimization problem, but my kids notice the faster speed when they visit.

Call support was no help. Every suggestion failed.

I have an additional issue that I haven't seen explicitly addressed. I frequently have incoming calls routed directly to voicemail after what seems to be a futile attempt by the T-Mobile network to locate my phone. It takes about 30 seconds for this to happen. The voicemail notification may be quick, or it may take a while to appear. This can happen when in cellular or Wi-Fi mode. It might be after a phone directed optimization takes place, but no way to know for certain.

Being old, I don't game or watch videos (other than news or weather), so my phone is primarily for communication. Since it often fails to communicate, I consider it defective and subject to replacement under my T-Mobile warranty. The problem is getting a replacement phone that doesn't have the problem.

To be certain to remedy this issue on T-Mobile, that means an iPhone.

Since this problem seems to be exclusive to T-Mobile, it is actually cheaper for me to switch to Spectrum Mobile using the $14 per Gig plan. 

If T-Mobile doesn’t have a solution soon, I may have to switch.