i had a long reply typed out earlier, but missed clicking to post it ..wrcy896 covered most of it.
if you're logged into the main page (it's separate from the forum, last i checked, but you can use the same login & password for both), if a repeater is listed as open, it should show the input and output frequencies (which you'll equate to channels on the radio), and input and output tones (which you'll take to the tone charts in the manual to see which code you need to enter in the menu). the 'repeater' channels on the radio are set up with the appropriate offset between transmit and receive.
for example, if i want to use an open repeater with an output of 462.625, input of 467.625, and input/output tones of 141.3: the table on page 27 shows that to be channel 18RP, so i'll tune to that then, go to the table on page 28 and look for 141.3 (it's ctcss) and find this equates to ctcss code 22 with the radio on 18rp, go into the menu setting for tones, select CT, select code 22, and save. mxt275 is online in pdf form here: https://midlandusa.com/wp-content/uploads/2017/10/MXT275-Owners-Manual.pdf
