How do I manually use cellular if my wifi or iPhone connection is too weak. Also, why does the watch not use cellular when iPhone is in airplane mode and wifi is off on watch? Why am i paying for it???

I can pick up cellular just fine when I shut down the connections from my iPhone, airplane mode is the easy way to do this for testing. Your watch and phone may be set to follow each other in airplane mode.

Check (iPhone) Watch app / My Watch / General / Airplane Mode. You don’t say why WiFi is not performing well (low signal around the house, or poor bandwidth to the outside world) but you will probably also need to turn off WiFi on your watch, or it will try to use WiFi when you test. If this doesn’t solve your problem check the cellular signal strength around your home, go outside and away from your iPhone and WiFi if necessary where you have a strong signal which connects ok then move back checking the signal showing on the watch.

I had time for a few tests today. I think we know how to do what you want, first the observed behavior.

• If I put the iPhone to airplane mode and leave all connectivity enabled on my watch, the watch hops onto the office WiFi. (re-enable phone)
• If I disable WiFi on the watch the watch remains connected by Bluetooth.
• If I use iPhone Control Center to disable Bluetooth the watch Bluetooth connection is privileged and remains active. This is by design, introduced about iOS 9.
• if I go into iPhone Settings and turn Bluetooth completely off the watch connects over cellular (WiFi on watch is still off from test #2).

So... it looks like you should try turning off Bluetooth in your iPhone Settings menu rather than just Control Center. Then you can turn off WiFi at the watch Control Center so it doesn’t see your router. Unfortunately there is no way I can see to disable the watch-phone Bluetooth link from the watch itself.