Hi,
First of all a simple test: Power off the router, the printer, the iPhone, and all computers and other devices. Wait two to three minutes. Power on the router. Wait another two to three minutes. Power on the printer. Wait a minute or two. Power on the iPhone. Finally, power on all computers and other devices. Any change?
The Brother MFC-L2700DW is mentioned in About AirPrint - Apple Support as a printer supporting the AirPrint technology. AirPrint should be enabled by default on the MFC-L2700DW (https://download.brother.com/welcome/doc100128/cv_hll2340dw_eng_ap_i.pdf#G7.1001613), but you may want to check that it has not been disabled for some reason.
Also, the router must allow Bonjour. Should be allowed by default in most cases, but may be worth checking.
The MFC-L2700DW appears to connect using 2.4 GHz only (https://help.brother-usa.com/app/answers/detail/a_id/73618/~/compatibility-with-5-ghz%2C-dual-band-wireless-router%2C-or-mesh-networks). In order for AirPrint to work, a printer and an iPhone must be connected to the same network. You may want to check this, and that 2.4 and 5 GHz bands do not lead to any difficulties (depending on router settings).