I don't see a usable possibility to have a static ip address with a mobile internet connection. The mobile device must give the RasPi a default route with its gateway. The gateway isn't static so you really should use DHCP.
With Raspbian or Raspberry Pi OS accepting network settings from a DHCP server (your Android cell phone) is working out of the box with ...
User error :P Hub unplugged downstream.
So my Raspberry Pi was connected to the a switch, that switch was connected to the wall jack, going to the network closet in the house. It was unplugged there at the router.
If you have little control over the network from using the mobile access point, you can create a script to check the wlan you are connected to and set a route by putting the script in /usr/lib/dhcpcd/dhcpcd-hooks/
You can get the connected network from
and the set the IP address etc within this script. In my mind, this is a little messy ...
You tried to use /etc/dhcpd.conf. It may be a typo, but have you checked /etc/dhcpcd.conf (different from the one you mentioned)?
If you have a static ip configured there this might explain what's happening here.
Roy Marples maintains the dhcpcd network package and the dhcpcd-ui for config - details are here on his site. Maybe asking on the mailing list could turn something up if you do not want to fork/edit the code yourself?
It may help others to edit your question to detail your needs a bit more (e.g. explain what you need it to do)?