Since very few cellphones nowadays have external antennas, and they are not Android but are 2G only, a cellular repeater or femtocell at 20m elevation is recommended.
It must match the cellular voice and data frequency bands used by your provider, wherever you are, and may require regional regulatory approval. They will work, as adding another cellphone (Android or otherwise) may or may not work, especially since the sensitivity and rejection figures needed to determine if that second phone would work well are rarely published. A repeater also eliminates the need for another cellphone and another SIM card.
An active repeater adds signal gain, whereas a passive repeater doesn't. Therefore, an active repeater is preferred.
The following picture shows two antennas coupled together; if there's an active repeater between the antennas instead of just cable, that would be ideal and much more likely to work with no problems, rather than an unamplified passive approach.
If there is no femtocell or cellular repeater approved, let's talk about external antennas coupled through low-loss cable to form a passive repeater.
L50 ohm LMR400 cable would be my choice for cable; see https://info.wilsonpro.com/ty-installationguide/ and https://www.wilsonamplifiers.com/blog/understanding-coaxial-cables-the-complete-guide for more.
There are apps for your phone which can provide the location of the cell towers you connect to at that 20 meter height. A Yagi-Uda or other directional antenna can be pointed at that cell tower, and a cable dropped from that position to your cellphone. However, that depends on knowing what frequency bands your cellphone carrier uses, as directional antennas are very frequency-specific.
Both of those suggestions let you continue to use your existing iPhone, and its Bluetooth connectivity, without buying another SIM card and paying for another cellular device every month.
